;;; css-mode.el --- Major mode to edit CSS files  -*- lexical-binding: t -*-

;; Copyright (C) 2006-2017 Free Software Foundation, Inc.

;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
;; Maintainer: Simen Heggestøyl <simenheg@gmail.com>
;; Keywords: hypermedia

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; Yet another CSS mode.

;;; Todo:

;; - filling code with auto-fill-mode
;; - fix font-lock errors with multi-line selectors

;;; Code:

(require 'seq)
(require 'sgml-mode)
(require 'smie)
(require 'eww)

(defgroup css nil
  "Cascading Style Sheets (CSS) editing mode."
  :group 'languages)

(defconst css-pseudo-class-ids
  '("active" "checked" "disabled" "empty" "enabled" "first"
    "first-child" "first-of-type" "focus" "hover" "indeterminate" "lang"
    "last-child" "last-of-type" "left" "link" "not" "nth-child"
    "nth-last-child" "nth-last-of-type" "nth-of-type" "only-child"
    "only-of-type" "right" "root" "target" "visited")
  "Identifiers for pseudo-classes.")

(defconst css-pseudo-element-ids
  '("after" "before" "first-letter" "first-line")
  "Identifiers for pseudo-elements.")

(defconst css-at-ids
  '("charset" "font-face" "import" "keyframes" "media" "namespace"
    "page" "supports")
  "Identifiers that appear in the form @foo.")

(defconst scss-at-ids
  '("at-root" "content" "debug" "each" "else" "else if" "error" "extend"
    "for" "function" "if" "import" "include" "mixin" "return" "warn"
    "while")
  "Additional identifiers that appear in the form @foo in SCSS.")

(defvar css--at-ids css-at-ids
  "List of at-rules for the current mode.")
(make-variable-buffer-local 'css--at-ids)

(defconst css-bang-ids
  '("important")
  "Identifiers that appear in the form !foo.")

(defconst scss-bang-ids
  '("default" "global" "optional")
  "Additional identifiers that appear in the form !foo in SCSS.")

(defvar css--bang-ids css-bang-ids
  "List of bang-rules for the current mode.")
(make-variable-buffer-local 'css--bang-ids)

(defconst css-descriptor-ids
  '("ascent" "baseline" "bbox" "cap-height" "centerline" "definition-src"
    "descent" "font-family" "font-size" "font-stretch" "font-style"
    "font-variant" "font-weight" "mathline" "panose-1" "slope" "src" "stemh"
    "stemv" "topline" "unicode-range" "units-per-em" "widths" "x-height")
  "Identifiers for font descriptors.")

(defconst css-media-ids
  '("all" "aural" "bitmap" "continuous" "grid" "paged" "static" "tactile"
    "visual")
  "Identifiers for types of media.")

(defconst css-property-alist
  ;; CSS 2.1 properties (http://www.w3.org/TR/CSS21/propidx.html).
  ;;
  ;; Properties duplicated by any of the CSS3 modules below have been
  ;; removed.
  '(("azimuth" angle "left-side" "far-left" "left" "center-left"
     "center" "center-right" "right" "far-right" "right-side" "behind"
     "leftwards" "rightwards")
    ("border-collapse" "collapse" "separate")
    ("border-spacing" length)
    ("bottom" length percentage "auto")
    ("caption-side" "top" "bottom")
    ("clear" "none" "left" "right" "both")
    ("clip" shape "auto")
    ("content" "normal" "none" string uri counter "attr()"
     "open-quote" "close-quote" "no-open-quote" "no-close-quote")
    ("counter-increment" identifier integer "none")
    ("counter-reset" identifier integer "none")
    ("cue" cue-before cue-after)
    ("cue-after" uri "none")
    ("cue-before" uri "none")
    ("direction" "ltr" "rtl")
    ("display" "inline" "block" "list-item" "inline-block" "table"
     "inline-table" "table-row-group" "table-header-group"
     "table-footer-group" "table-row" "table-column-group"
     "table-column" "table-cell" "table-caption" "none"
     ;; CSS Flexible Box Layout Module Level 1
     ;; (https://www.w3.org/TR/css3-flexbox/#valdef-display-flex)
     "flex" "inline-flex")
    ("elevation" angle "below" "level" "above" "higher" "lower")
    ("empty-cells" "show" "hide")
    ("float" "left" "right" "none")
    ("height" length percentage "auto")
    ("left" length percentage "auto")
    ("line-height" "normal" number length percentage)
    ("list-style" list-style-type list-style-position
     list-style-image)
    ("list-style-image" uri "none")
    ("list-style-position" "inside" "outside")
    ("list-style-type" "disc" "circle" "square" "decimal"
     "decimal-leading-zero" "lower-roman" "upper-roman" "lower-greek"
     "lower-latin" "upper-latin" "armenian" "georgian" "lower-alpha"
     "upper-alpha" "none")
    ("margin" margin-width)
    ("margin-bottom" margin-width)
    ("margin-left" margin-width)
    ("margin-right" margin-width)
    ("margin-top" margin-width)
    ("max-height" length percentage "none")
    ("max-width" length percentage "none")
    ("min-height" length percentage)
    ("min-width" length percentage)
    ("padding" padding-width)
    ("padding-bottom" padding-width)
    ("padding-left" padding-width)
    ("padding-right" padding-width)
    ("padding-top" padding-width)
    ("page-break-after" "auto" "always" "avoid" "left" "right")
    ("page-break-before" "auto" "always" "avoid" "left" "right")
    ("page-break-inside" "avoid" "auto")
    ("pause" time percentage)
    ("pause-after" time percentage)
    ("pause-before" time percentage)
    ("pitch" frequency "x-low" "low" "medium" "high" "x-high")
    ("pitch-range" number)
    ("play-during" uri "mix" "repeat" "auto" "none")
    ("position" "static" "relative" "absolute" "fixed")
    ("quotes" string "none")
    ("richness" number)
    ("right" length percentage "auto")
    ("speak" "normal" "none" "spell-out")
    ("speak-header" "once" "always")
    ("speak-numeral" "digits" "continuous")
    ("speak-punctuation" "code" "none")
    ("speech-rate" number "x-slow" "slow" "medium" "fast" "x-fast"
     "faster" "slower")
    ("stress" number)
    ("table-layout" "auto" "fixed")
    ("top" length percentage "auto")
    ("unicode-bidi" "normal" "embed" "bidi-override")
    ("vertical-align" "baseline" "sub" "super" "top" "text-top"
     "middle" "bottom" "text-bottom" percentage length)
    ("visibility" "visible" "hidden" "collapse")
    ("voice-family" specific-voice generic-voice specific-voice
     generic-voice)
    ("volume" number percentage "silent" "x-soft" "soft" "medium"
     "loud" "x-loud")
    ("width" length percentage "auto")
    ("z-index" "auto" integer)

    ;; CSS Animations
    ;; (http://www.w3.org/TR/css3-animations/#property-index)
    ("animation" single-animation-name time single-timing-function
     single-animation-iteration-count single-animation-direction
     single-animation-fill-mode single-animation-play-state)
    ("animation-delay" time)
    ("animation-direction" single-animation-direction)
    ("animation-duration" time)
    ("animation-fill-mode" single-animation-fill-mode)
    ("animation-iteration-count" single-animation-iteration-count)
    ("animation-name" single-animation-name)
    ("animation-play-state" single-animation-play-state)
    ("animation-timing-function" single-timing-function)

    ;; CSS Backgrounds and Borders Module Level 3
    ;; (http://www.w3.org/TR/css3-background/#property-index)
    ("background" bg-layer final-bg-layer)
    ("background-attachment" attachment)
    ("background-clip" box)
    ("background-color" color)
    ("background-image" bg-image)
    ("background-origin" box)
    ("background-position" position)
    ("background-repeat" repeat-style)
    ("background-size" bg-size)
    ("border" line-width line-style color)
    ("border-bottom" line-width line-style color)
    ("border-bottom-color" color)
    ("border-bottom-left-radius" length percentage)
    ("border-bottom-right-radius" length percentage)
    ("border-bottom-style" line-style)
    ("border-bottom-width" line-width)
    ("border-color" color)
    ("border-image" border-image-source border-image-slice
     border-image-width border-image-outset border-image-repeat)
    ("border-image-outset" length number)
    ("border-image-repeat" "stretch" "repeat" "round" "space")
    ("border-image-slice" number percentage "fill")
    ("border-image-source" "none" image)
    ("border-image-width" length percentage number "auto")
    ("border-left" line-width line-style color)
    ("border-left-color" color)
    ("border-left-style" line-style)
    ("border-left-width" line-width)
    ("border-radius" length percentage)
    ("border-right" line-width line-style color)
    ("border-right-color" color)
    ("border-right-style" line-style)
    ("border-right-width" line-width)
    ("border-style" line-style)
    ("border-top" line-width line-style color)
    ("border-top-color" color)
    ("border-top-left-radius" length percentage)
    ("border-top-right-radius" length percentage)
    ("border-top-style" line-style)
    ("border-top-width" line-width)
    ("border-width" line-width)
    ("box-shadow" "none" shadow)

    ;; CSS Basic User Interface Module Level 3 (CSS3 UI)
    ;; (http://www.w3.org/TR/css3-ui/#property-index)
    ("box-sizing" "content-box" "border-box")
    ("caret-color" "auto" color)
    ("cursor" uri x y "auto" "default" "none" "context-menu" "help"
     "pointer" "progress" "wait" "cell" "crosshair" "text"
     "vertical-text" "alias" "copy" "move" "no-drop" "not-allowed"
     "grab" "grabbing" "e-resize" "n-resize" "ne-resize" "nw-resize"
     "s-resize" "se-resize" "sw-resize" "w-resize" "ew-resize"
     "ns-resize" "nesw-resize" "nwse-resize" "col-resize" "row-resize"
     "all-scroll" "zoom-in" "zoom-out")
    ("nav-down" "auto" id "current" "root" target-name)
    ("nav-left" "auto" id "current" "root" target-name)
    ("nav-right" "auto" id "current" "root" target-name)
    ("nav-up" "auto" id "current" "root" target-name)
    ("outline" outline-color outline-style outline-width)
    ("outline-color" color "invert")
    ("outline-offset" length)
    ("outline-style" "auto" border-style)
    ("outline-width" border-width)
    ("resize" "none" "both" "horizontal" "vertical")
    ("text-overflow" "clip" "ellipsis" string)

    ;; CSS Color Module Level 3
    ;; (http://www.w3.org/TR/css3-color/#property)
    ("color" color)
    ("opacity" alphavalue)

    ;; CSS Flexible Box Layout Module Level 1
    ;; (http://www.w3.org/TR/css-flexbox-1/#property-index)
    ("align-content" "flex-start" "flex-end" "center" "space-between"
     "space-around" "stretch")
    ("align-items" "flex-start" "flex-end" "center" "baseline"
     "stretch")
    ("align-self" "auto" "flex-start" "flex-end" "center" "baseline"
     "stretch")
    ("flex" "none" flex-grow flex-shrink flex-basis)
    ("flex-basis" "auto" "content" width)
    ("flex-direction" "row" "row-reverse" "column" "column-reverse")
    ("flex-flow" flex-direction flex-wrap)
    ("flex-grow" number)
    ("flex-shrink" number)
    ("flex-wrap" "nowrap" "wrap" "wrap-reverse")
    ("justify-content" "flex-start" "flex-end" "center"
     "space-between" "space-around")
    ("order" integer)

    ;; CSS Fonts Module Level 3
    ;; (http://www.w3.org/TR/css3-fonts/#property-index)
    ("font" font-style font-variant-css21 font-weight font-stretch
     font-size line-height font-family "caption" "icon" "menu"
     "message-box" "small-caption" "status-bar")
    ("font-family" family-name generic-family)
    ("font-feature-settings" "normal" feature-tag-value)
    ("font-kerning" "auto" "normal" "none")
    ("font-language-override" "normal" string)
    ("font-size" absolute-size relative-size length percentage)
    ("font-size-adjust" "none" number)
    ("font-stretch" "normal" "ultra-condensed" "extra-condensed"
     "condensed" "semi-condensed" "semi-expanded" "expanded"
     "extra-expanded" "ultra-expanded")
    ("font-style" "normal" "italic" "oblique")
    ("font-synthesis" "none" "weight" "style")
    ("font-variant" "normal" "none" common-lig-values
     discretionary-lig-values historical-lig-values
     contextual-alt-values "stylistic()" "historical-forms"
     "styleset()" "character-variant()" "swash()" "ornaments()"
     "annotation()" "small-caps" "all-small-caps" "petite-caps"
     "all-petite-caps" "unicase" "titling-caps" numeric-figure-values
     numeric-spacing-values numeric-fraction-values "ordinal"
     "slashed-zero" east-asian-variant-values east-asian-width-values
     "ruby")
    ("font-variant-alternates" "normal" "stylistic()"
     "historical-forms" "styleset()" "character-variant()" "swash()"
     "ornaments()" "annotation()")
    ("font-variant-caps" "normal" "small-caps" "all-small-caps"
     "petite-caps" "all-petite-caps" "unicase" "titling-caps")
    ("font-variant-east-asian" "normal" east-asian-variant-values
     east-asian-width-values "ruby")
    ("font-variant-ligatures" "normal" "none" common-lig-values
     discretionary-lig-values historical-lig-values
     contextual-alt-values)
    ("font-variant-numeric" "normal" numeric-figure-values
     numeric-spacing-values numeric-fraction-values "ordinal"
     "slashed-zero")
    ("font-variant-position" "normal" "sub" "super")
    ("font-weight" "normal" "bold" "bolder" "lighter" "100" "200"
     "300" "400" "500" "600" "700" "800" "900")

    ;; CSS Fragmentation Module Level 3
    ;; (https://www.w3.org/TR/css-break-3/#property-index)
    ("box-decoration-break" "slice" "clone")
    ("break-after" "auto" "avoid" "avoid-page" "page" "left" "right"
     "recto" "verso" "avoid-column" "column" "avoid-region" "region")
    ("break-before" "auto" "avoid" "avoid-page" "page" "left" "right"
     "recto" "verso" "avoid-column" "column" "avoid-region" "region")
    ("break-inside" "auto" "avoid" "avoid-page" "avoid-column"
     "avoid-region")
    ("orphans" integer)
    ("widows" integer)

    ;; CSS Multi-column Layout Module
    ;; (https://www.w3.org/TR/css3-multicol/#property-index)
    ;; "break-after", "break-before", and "break-inside" are left out
    ;; below, because they're already included in CSS Fragmentation
    ;; Module Level 3.
    ("column-count" integer "auto")
    ("column-fill" "auto" "balance")
    ("column-gap" length "normal")
    ("column-rule" column-rule-width column-rule-style
     column-rule-color "transparent")
    ("column-rule-color" color)
    ("column-rule-style" border-style)
    ("column-rule-width" border-width)
    ("column-span" "none" "all")
    ("column-width" length "auto")
    ("columns" column-width column-count)

    ;; CSS Overflow Module Level 3
    ;; (http://www.w3.org/TR/css-overflow-3/#property-index)
    ("max-lines" "none" integer)
    ("overflow" "visible" "hidden" "scroll" "auto" "paged-x" "paged-y"
     "paged-x-controls" "paged-y-controls" "fragments")
    ("overflow-x" "visible" "hidden" "scroll" "auto" "paged-x"
     "paged-y" "paged-x-controls" "paged-y-controls" "fragments")
    ("overflow-y" "visible" "hidden" "scroll" "auto" "paged-x"
     "paged-y" "paged-x-controls" "paged-y-controls" "fragments")

    ;; CSS Text Decoration Module Level 3
    ;; (http://dev.w3.org/csswg/css-text-decor-3/#property-index)
    ("text-decoration" text-decoration-line text-decoration-style
     text-decoration-color)
    ("text-decoration-color" color)
    ("text-decoration-line" "none" "underline" "overline"
     "line-through" "blink")
    ("text-decoration-skip" "none" "objects" "spaces" "ink" "edges"
     "box-decoration")
    ("text-decoration-style" "solid" "double" "dotted" "dashed"
     "wavy")
    ("text-emphasis" text-emphasis-style text-emphasis-color)
    ("text-emphasis-color" color)
    ("text-emphasis-position" "over" "under" "right" "left")
    ("text-emphasis-style" "none" "filled" "open" "dot" "circle"
     "double-circle" "triangle" "sesame" string)
    ("text-shadow" "none" length color)
    ("text-underline-position" "auto" "under" "left" "right")

    ;; CSS Text Module Level 3
    ;; (http://www.w3.org/TR/css3-text/#property-index)
    ("hanging-punctuation" "none" "first" "force-end" "allow-end"
     "last")
    ("hyphens" "none" "manual" "auto")
    ("letter-spacing" "normal" length)
    ("line-break" "auto" "loose" "normal" "strict")
    ("overflow-wrap" "normal" "break-word")
    ("tab-size" integer length)
    ("text-align" "start" "end" "left" "right" "center" "justify"
     "match-parent")
    ("text-align-last" "auto" "start" "end" "left" "right" "center"
     "justify")
    ("text-indent" length percentage)
    ("text-justify" "auto" "none" "inter-word" "distribute")
    ("text-transform" "none" "capitalize" "uppercase" "lowercase"
     "full-width")
    ("white-space" "normal" "pre" "nowrap" "pre-wrap" "pre-line")
    ("word-break" "normal" "keep-all" "break-all")
    ("word-spacing" "normal" length percentage)
    ("word-wrap" "normal" "break-word")

    ;; CSS Transforms Module Level 1
    ;; (http://www.w3.org/TR/css3-2d-transforms/#property-index)
    ("backface-visibility" "visible" "hidden")
    ("perspective" "none" length)
    ("perspective-origin" "left" "center" "right" "top" "bottom"
     percentage length)
    ("transform" "none" transform-list)
    ("transform-origin" "left" "center" "right" "top" "bottom"
     percentage length)
    ("transform-style" "flat" "preserve-3d")

    ;; CSS Transitions
    ;; (http://www.w3.org/TR/css3-transitions/#property-index)
    ("transition" single-transition)
    ("transition-delay" time)
    ("transition-duration" time)
    ("transition-property" "none" single-transition-property "all")
    ("transition-timing-function" single-transition-timing-function)

    ;; CSS Will Change Module Level 1
    ;; (https://www.w3.org/TR/css-will-change-1/#property-index)
    ("will-change" "auto" animateable-feature)

    ;; Filter Effects Module Level 1
    ;; (http://www.w3.org/TR/filter-effects/#property-index)
    ("color-interpolation-filters" "auto" "sRGB" "linearRGB")
    ("filter" "none" filter-function-list)
    ("flood-color" color)
    ("flood-opacity" number percentage)
    ("lighting-color" color))
  "Identifiers for properties and their possible values.
The CAR of each entry is the name of a property, while the CDR is
a list of possible values for that property.  String values in
the CDRs represent literal values, while symbols represent one of
the value classes found in `css-value-class-alist'.  If a symbol
is not found in `css-value-class-alist', it's interpreted as a
reference back to one of the properties in this list.  Some
symbols, such as `number' or `identifier', don't produce any
further value candidates, since that list would be infinite.")

(defconst css-property-ids
  (mapcar #'car css-property-alist)
  "Identifiers for properties.")

(defconst css-value-class-alist
  '((absolute-size
     "xx-small" "x-small" "small" "medium" "large" "x-large"
     "xx-large")
    (alphavalue number)
    (angle "calc()")
    (animateable-feature "scroll-position" "contents" custom-ident)
    (attachment "scroll" "fixed" "local")
    (bg-image image "none")
    (bg-layer bg-image position repeat-style attachment box)
    (bg-size length percentage "auto" "cover" "contain")
    (box "border-box" "padding-box" "content-box")
    (color
     "rgb()" "rgba()" "hsl()" "hsla()" named-color "transparent"
     "currentColor")
    (common-lig-values "common-ligatures" "no-common-ligatures")
    (contextual-alt-values "contextual" "no-contextual")
    (counter "counter()" "counters()")
    (discretionary-lig-values
     "discretionary-ligatures" "no-discretionary-ligatures")
    (east-asian-variant-values
     "jis78" "jis83" "jis90" "jis04" "simplified" "traditional")
    (east-asian-width-values "full-width" "proportional-width")
    (family-name "Courier" "Helvetica" "Times")
    (feature-tag-value string integer "on" "off")
    (filter-function
     "blur()" "brightness()" "contrast()" "drop-shadow()"
     "grayscale()" "hue-rotate()" "invert()" "opacity()" "sepia()"
     "saturate()")
    (filter-function-list filter-function uri)
    (final-bg-layer
     bg-image position repeat-style attachment box color)
    (font-variant-css21 "normal" "small-caps")
    (frequency "calc()")
    (generic-family
     "serif" "sans-serif" "cursive" "fantasy" "monospace")
    (generic-voice "male" "female" "child")
    (gradient
     linear-gradient radial-gradient repeating-linear-gradient
     repeating-radial-gradient)
    (historical-lig-values
     "historical-ligatures" "no-historical-ligatures")
    (image uri image-list element-reference gradient)
    (image-list "image()")
    (integer "calc()")
    (length "calc()" number)
    (line-height "normal" number length percentage)
    (line-style
     "none" "hidden" "dotted" "dashed" "solid" "double" "groove"
     "ridge" "inset" "outset")
    (line-width length "thin" "medium" "thick")
    (linear-gradient "linear-gradient()")
    (margin-width "auto" length percentage)
    (named-color
     "aliceblue" "antiquewhite" "aqua" "aquamarine" "azure" "beige"
     "bisque" "black" "blanchedalmond" "blue" "blueviolet" "brown"
     "burlywood" "cadetblue" "chartreuse" "chocolate" "coral"
     "cornflowerblue" "cornsilk" "crimson" "cyan" "darkblue"
     "darkcyan" "darkgoldenrod" "darkgray" "darkgreen" "darkkhaki"
     "darkmagenta" "darkolivegreen" "darkorange" "darkorchid"
     "darkred" "darksalmon" "darkseagreen" "darkslateblue"
     "darkslategray" "darkturquoise" "darkviolet" "deeppink"
     "deepskyblue" "dimgray" "dodgerblue" "firebrick" "floralwhite"
     "forestgreen" "fuchsia" "gainsboro" "ghostwhite" "gold"
     "goldenrod" "gray" "green" "greenyellow" "honeydew" "hotpink"
     "indianred" "indigo" "ivory" "khaki" "lavender" "lavenderblush"
     "lawngreen" "lemonchiffon" "lightblue" "lightcoral" "lightcyan"
     "lightgoldenrodyellow" "lightgray" "lightgreen" "lightpink"
     "lightsalmon" "lightseagreen" "lightskyblue" "lightslategray"
     "lightsteelblue" "lightyellow" "lime" "limegreen" "linen"
     "magenta" "maroon" "mediumaquamarine" "mediumblue" "mediumorchid"
     "mediumpurple" "mediumseagreen" "mediumslateblue"
     "mediumspringgreen" "mediumturquoise" "mediumvioletred"
     "midnightblue" "mintcream" "mistyrose" "moccasin" "navajowhite"
     "navy" "oldlace" "olive" "olivedrab" "orange" "orangered"
     "orchid" "palegoldenrod" "palegreen" "paleturquoise"
     "palevioletred" "papayawhip" "peachpuff" "peru" "pink" "plum"
     "powderblue" "purple" "rebeccapurple" "red" "rosybrown"
     "royalblue" "saddlebrown" "salmon" "sandybrown" "seagreen"
     "seashell" "sienna" "silver" "skyblue" "slateblue" "slategray"
     "snow" "springgreen" "steelblue" "tan" "teal" "thistle" "tomato"
     "turquoise" "violet" "wheat" "white" "whitesmoke" "yellow"
     "yellowgreen")
    (number "calc()")
    (numeric-figure-values "lining-nums" "oldstyle-nums")
    (numeric-fraction-values "diagonal-fractions" "stacked-fractions")
    (numeric-spacing-values "proportional-nums" "tabular-nums")
    (padding-width length percentage)
    (position
     "left" "center" "right" "top" "bottom" percentage length)
    (radial-gradient "radial-gradient()")
    (relative-size "larger" "smaller")
    (repeat-style
     "repeat-x" "repeat-y" "repeat" "space" "round" "no-repeat")
    (repeating-linear-gradient "repeating-linear-gradient()")
    (repeating-radial-gradient "repeating-radial-gradient()")
    (shadow "inset" length color)
    (shape "rect()")
    (single-animation-direction
     "normal" "reverse" "alternate" "alternate-reverse")
    (single-animation-fill-mode "none" "forwards" "backwards" "both")
    (single-animation-iteration-count "infinite" number)
    (single-animation-name "none" identifier)
    (single-animation-play-state "running" "paused")
    (single-timing-function single-transition-timing-function)
    (single-transition
     "none" single-transition-property time
     single-transition-timing-function)
    (single-transition-property "all" identifier)
    (single-transition-timing-function
     "ease" "linear" "ease-in" "ease-out" "ease-in-out" "step-start"
     "step-end" "steps()" "cubic-bezier()")
    (specific-voice identifier)
    (target-name string)
    (time "calc()")
    (transform-list
     "matrix()" "translate()" "translateX()" "translateY()" "scale()"
     "scaleX()" "scaleY()" "rotate()" "skew()" "skewX()" "skewY()"
     "matrix3d()" "translate3d()" "translateZ()" "scale3d()"
     "scaleZ()" "rotate3d()" "rotateX()" "rotateY()" "rotateZ()"
     "perspective()")
    (uri "url()")
    (width length percentage "auto")
    (x number)
    (y number))
  "Property value classes and their values.
The format is similar to that of `css-property-alist', except
that the CARs aren't actual CSS properties, but rather a name for
a class of values, and that symbols in the CDRs always refer to
other entries in this list, not to properties.

The following classes have been left out above because they
cannot be completed sensibly: `custom-ident',
`element-reference', `id', `identifier', `percentage', and
`string'.")

(defcustom css-electric-keys '(?\} ?\;) ;; '()
  "Self inserting keys which should trigger re-indentation."
  :version "22.2"
  :type '(repeat character)
  :options '((?\} ?\;))
  :group 'css)

(defvar css-mode-syntax-table
  (let ((st (make-syntax-table)))
    ;; C-style comments.
    (modify-syntax-entry ?/ ". 14" st)
    (modify-syntax-entry ?* ". 23b" st)
    ;; Strings.
    (modify-syntax-entry ?\" "\"" st)
    (modify-syntax-entry ?\' "\"" st)
    ;; Blocks.
    (modify-syntax-entry ?\{ "(}" st)
    (modify-syntax-entry ?\} "){" st)
    ;; Args in url(...) thingies and other "function calls".
    (modify-syntax-entry ?\( "()" st)
    (modify-syntax-entry ?\) ")(" st)
    ;; To match attributes in selectors.
    (modify-syntax-entry ?\[ "(]" st)
    (modify-syntax-entry ?\] ")[" st)
    ;; Special chars that sometimes come at the beginning of words.
    (modify-syntax-entry ?@ "'" st)
    ;; (modify-syntax-entry ?: "'" st)
    (modify-syntax-entry ?# "'" st)
    ;; Distinction between words and symbols.
    (modify-syntax-entry ?- "_" st)
    st))

(defvar css-mode-map
  (let ((map (make-sparse-keymap)))
    (define-key map [remap info-lookup-symbol] 'css-lookup-symbol)
    map)
  "Keymap used in `css-mode'.")

(eval-and-compile
  (defconst css--uri-re
    (concat
     "url\\((\\)[[:space:]]*\\(?:\\\\.\\|[^()[:space:]\n'\"]\\)+"
     "[[:space:]]*\\()\\)")))

(defconst css-syntax-propertize-function
  (syntax-propertize-rules
   (css--uri-re (1 "|") (2 "|"))))

(defconst css-escapes-re
  "\\\\\\(?:[^\000-\037\177]\\|[0-9a-fA-F]+[ \n\t\r\f]?\\)")
(defconst css-nmchar-re (concat "\\(?:[-[:alnum:]]\\|" css-escapes-re "\\)"))
(defconst css-nmstart-re (concat "\\(?:[[:alpha:]]\\|" css-escapes-re "\\)"))
(defconst css-ident-re ;; (concat css-nmstart-re css-nmchar-re "*")
  ;; Apparently, "at rules" names can start with a dash, e.g. @-moz-keyframes.
  (concat css-nmchar-re "+"))
(defconst css-proprietary-nmstart-re ;; Vendor-specific properties.
  (concat "[-_]" (regexp-opt '("ms" "moz" "o" "khtml" "webkit")) "-"))
(defconst css-name-re (concat css-nmchar-re "+"))

(defconst scss--hash-re "#\\(?:{[$-_[:alnum:]]+}\\|[[:alnum:]]+\\)")

(defface css-selector '((t :inherit font-lock-function-name-face))
  "Face to use for selectors."
  :group 'css)
(defface css-property '((t :inherit font-lock-variable-name-face))
  "Face to use for properties."
  :group 'css)
(defface css-proprietary-property '((t :inherit (css-property italic)))
  "Face to use for vendor-specific properties.")

(defun css--font-lock-keywords (&optional sassy)
  `((,(concat "!\\s-*" (regexp-opt css--bang-ids))
     (0 font-lock-builtin-face))
    ;; Atrules keywords.  IDs not in css-at-ids are valid (ignored).
    ;; In fact the regexp should probably be
    ;; (,(concat "\\(@" css-ident-re "\\)\\([ \t\n][^;{]*\\)[;{]")
    ;;  (1 font-lock-builtin-face))
    ;; Since "An at-rule consists of everything up to and including the next
    ;; semicolon (;) or the next block, whichever comes first."
    (,(concat "@" css-ident-re) (0 font-lock-builtin-face))
    ;; Variables.
    (,(concat "--" css-ident-re) (0 font-lock-variable-name-face))
    ;; Selectors.
    ;; Allow plain ":root" as a selector.
    ("^[ \t]*\\(:root\\)\\(?:[\n \t]*\\)*{" (1 'css-selector keep))
    ;; FIXME: attribute selectors don't work well because they may contain
    ;; strings which have already been highlighted as f-l-string-face and
    ;; thus prevent this highlighting from being applied (actually now that
    ;; I use `keep' this should work better).  But really the part of the
    ;; selector between [...] should simply not be highlighted.
    (,(concat
       "^[ \t]*\\("
       (if (not sassy)
           ;; We don't allow / as first char, so as not to
           ;; take a comment as the beginning of a selector.
           "[^@/:{}() \t\n][^:{}()]+"
         ;; Same as for non-sassy except we do want to allow { and }
         ;; chars in selectors in the case of #{$foo}
         ;; variable interpolation!
         (concat "\\(?:" scss--hash-re
                 "\\|[^@/:{}() \t\n#]\\)"
                 "[^:{}()#]*\\(?:" scss--hash-re "[^:{}()#]*\\)*"))
       ;; Even though pseudo-elements should be prefixed by ::, a
       ;; single colon is accepted for backward compatibility.
       "\\(?:\\(:" (regexp-opt (append css-pseudo-class-ids
                                       css-pseudo-element-ids) t)
       "\\|\\::" (regexp-opt css-pseudo-element-ids t) "\\)"
       "\\(?:([^)]+)\\)?"
       (if (not sassy)
           "[^:{}()\n]*"
         (concat "[^:{}()\n#]*\\(?:" scss--hash-re "[^:{}()\n#]*\\)*"))
       "\\)*"
       "\\)\\(?:\n[ \t]*\\)*{")
     (1 'css-selector keep))
    ;; In the above rule, we allow the open-brace to be on some subsequent
    ;; line.  This will only work if we properly mark the intervening text
    ;; as being part of a multiline element (and even then, this only
    ;; ensures proper refontification, but not proper discovery).
    ("^[ \t]*{" (0 (save-excursion
                     (goto-char (match-beginning 0))
                     (skip-chars-backward " \n\t")
                     (put-text-property (point) (match-end 0)
                                        'font-lock-multiline t)
                     ;; No face.
                     nil)))
    ;; Properties.  Again, we don't limit ourselves to css-property-ids.
    (,(concat "\\(?:[{;]\\|^\\)[ \t]*\\("
              "\\(?:\\(" css-proprietary-nmstart-re "\\)\\|"
              css-nmstart-re "\\)" css-nmchar-re "*"
              "\\)\\s-*:")
     (1 (if (match-end 2) 'css-proprietary-property 'css-property)))
    ;; Make sure the parens in a url(...) expression receive the
    ;; default face. This is done because the parens may sometimes
    ;; receive generic string delimiter syntax (see
    ;; `css-syntax-propertize-function').
    (,css--uri-re
     (1 'default t) (2 'default t))))

(defvar css-font-lock-keywords (css--font-lock-keywords))

(defvar css-font-lock-defaults
  '(css-font-lock-keywords nil t))

(defcustom css-indent-offset 4
  "Basic size of one indentation step."
  :version "22.2"
  :type 'integer
  :safe 'integerp)

(defconst css-smie-grammar
  (smie-prec2->grammar
   (smie-precs->prec2 '((assoc ";") (assoc ",") (left ":")))))

(defun css-smie--forward-token ()
  (cond
   ((and (eq (char-before) ?\})
         (scss-smie--not-interpolation-p)
         ;; FIXME: If the next char is not whitespace, what should we do?
         (or (memq (char-after) '(?\s ?\t ?\n))
             (looking-at comment-start-skip)))
    (if (memq (char-after) '(?\s ?\t ?\n))
        (forward-char 1) (forward-comment 1))
    ";")
   ((progn (forward-comment (point-max))
           (looking-at "[;,:]"))
    (forward-char 1) (match-string 0))
   (t (smie-default-forward-token))))

(defun css-smie--backward-token ()
  (let ((pos (point)))
    (forward-comment (- (point)))
    (cond
     ;; FIXME: If the next char is not whitespace, what should we do?
     ((and (eq (char-before) ?\}) (scss-smie--not-interpolation-p)
           (> pos (point))) ";")
     ((memq (char-before) '(?\; ?\, ?\:))
      (forward-char -1) (string (char-after)))
     (t (smie-default-backward-token)))))

(defun css-smie-rules (kind token)
  (pcase (cons kind token)
    (`(:elem . basic) css-indent-offset)
    (`(:elem . arg) 0)
    (`(:list-intro . ,(or `";" `"")) t) ;"" stands for BOB (bug#15467).
    (`(:before . "{")
     (when (or (smie-rule-hanging-p) (smie-rule-bolp))
       (smie-backward-sexp ";")
       (smie-indent-virtual)))
    (`(:before . ,(or "{" "("))
     (if (smie-rule-hanging-p) (smie-rule-parent 0)))))

;;; Completion

(defun css--complete-property ()
  "Complete property at point."
  (save-excursion
    (let ((pos (point)))
      (skip-chars-backward "-[:alnum:]")
      (let ((start (point)))
        (skip-chars-backward " \t\r\n")
        (when (memq (char-before) '(?\{ ?\;))
          (list start pos css-property-ids))))))

(defun css--complete-bang-rule ()
  "Complete bang-rule at point."
  (save-excursion
    (let ((pos (point)))
      (skip-chars-backward "-[:alnum:]")
      (when (eq (char-before) ?\!)
        (list (point) pos css--bang-ids)))))

(defun css--complete-pseudo-element-or-class ()
  "Complete pseudo-element or pseudo-class at point."
  (save-excursion
    (let ((pos (point)))
      (skip-chars-backward "-[:alnum:]")
      (when (eq (char-before) ?\:)
        (list (point) pos
              (if (eq (char-before (- (point) 1)) ?\:)
                  css-pseudo-element-ids
                css-pseudo-class-ids))))))

(defun css--complete-at-rule ()
  "Complete at-rule (statement beginning with `@') at point."
  (save-excursion
    (let ((pos (point)))
      (skip-chars-backward "-[:alnum:]")
      (when (eq (char-before) ?\@)
        (list (point) pos css--at-ids)))))

(defvar css--property-value-cache
  (make-hash-table :test 'equal :size (length css-property-alist))
  "Cache of previously completed property values.")

(defun css--value-class-lookup (value-class)
  "Return a list of value completion candidates for VALUE-CLASS.
Completion candidates are looked up in `css-value-class-alist' by
the symbol VALUE-CLASS."
  (seq-uniq
   (seq-mapcat
    (lambda (value)
      (if (stringp value)
          (list value)
        (css--value-class-lookup value)))
    (cdr (assq value-class css-value-class-alist)))))

(defun css--property-values (property)
  "Return a list of value completion candidates for PROPERTY.
Completion candidates are looked up in `css-property-alist' by
the string PROPERTY."
  (or (gethash property css--property-value-cache)
      (let ((values
             (seq-uniq
              (seq-mapcat
               (lambda (value)
                 (if (stringp value)
                     (list value)
                   (or (css--value-class-lookup value)
                       (css--property-values (symbol-name value)))))
               (cdr (assoc property css-property-alist))))))
        (puthash property values css--property-value-cache))))

(defun css--complete-property-value ()
  "Complete property value at point."
  (let ((property
         (save-excursion
           (re-search-backward ":[^/]" (line-beginning-position) t)
           (let ((property-end (point)))
             (skip-chars-backward "-[:alnum:]")
             (let ((property (buffer-substring (point) property-end)))
               (car (member property css-property-ids)))))))
    (when property
      (let ((end (point)))
        (save-excursion
          (skip-chars-backward "[:graph:]")
          (list (point) end
                (append '("inherit" "initial" "unset")
                        (css--property-values property))))))))

(defvar css--html-tags (mapcar #'car html-tag-alist)
  "List of HTML tags.
Used to provide completion of HTML tags in selectors.")

(defvar css--nested-selectors-allowed nil
  "Non-nil if nested selectors are allowed in the current mode.")
(make-variable-buffer-local 'css--nested-selectors-allowed)

(defvar css-class-list-function #'ignore
  "Called to provide completions of class names.
This can be bound by buffers that are able to suggest class name
completions, such as HTML mode buffers.")

(defvar css-id-list-function #'ignore
  "Called to provide completions of IDs.
This can be bound by buffers that are able to suggest ID
completions, such as HTML mode buffers.")

(defun css--foreign-completions (extractor)
  "Return a list of completions provided by other buffers.
EXTRACTOR should be the name of a function that may be defined in
one or more buffers.  In each of the buffers where EXTRACTOR is
defined, EXTRACTOR is called and the results are accumulated into
a list of completions."
  (delete-dups
   (seq-mapcat
    (lambda (buf)
      (with-current-buffer buf
        (funcall (symbol-value extractor))))
    (buffer-list))))

(defun css--complete-selector ()
  "Complete part of a CSS selector at point."
  (when (or (= (nth 0 (syntax-ppss)) 0) css--nested-selectors-allowed)
    (let ((end (point)))
      (save-excursion
        (skip-chars-backward "-[:alnum:]")
        (let ((start-char (char-before)))
          (list
           (point) end
           (completion-table-dynamic
            (lambda (_)
              (cond
               ((eq start-char ?.)
                (css--foreign-completions 'css-class-list-function))
               ((eq start-char ?#)
                (css--foreign-completions 'css-id-list-function))
               (t css--html-tags))))))))))

(defun css-completion-at-point ()
  "Complete current symbol at point.
Currently supports completion of CSS properties, property values,
pseudo-elements, pseudo-classes, at-rules, and bang-rules."
  (or (css--complete-bang-rule)
      (css--complete-property-value)
      (css--complete-pseudo-element-or-class)
      (css--complete-at-rule)
      (seq-let (prop-beg prop-end prop-table) (css--complete-property)
        (seq-let (sel-beg sel-end sel-table) (css--complete-selector)
          (when (or prop-table sel-table)
            `(,@(if prop-table
                    (list prop-beg prop-end)
                  (list sel-beg sel-end))
              ,(completion-table-merge prop-table sel-table)))))))

;;;###autoload
(define-derived-mode css-mode prog-mode "CSS"
  "Major mode to edit Cascading Style Sheets."
  (setq-local font-lock-defaults css-font-lock-defaults)
  (setq-local comment-start "/*")
  (setq-local comment-start-skip "/\\*+[ \t]*")
  (setq-local comment-end "*/")
  (setq-local comment-end-skip "[ \t]*\\*+/")
  (setq-local syntax-propertize-function
              css-syntax-propertize-function)
  (setq-local fill-paragraph-function #'css-fill-paragraph)
  (setq-local adaptive-fill-function #'css-adaptive-fill)
  (setq-local add-log-current-defun-function #'css-current-defun-name)
  (smie-setup css-smie-grammar #'css-smie-rules
              :forward-token #'css-smie--forward-token
              :backward-token #'css-smie--backward-token)
  (setq-local electric-indent-chars
              (append css-electric-keys electric-indent-chars))
  (add-hook 'completion-at-point-functions
            #'css-completion-at-point nil 'local))

(defvar comment-continue)

(defun css-fill-paragraph (&optional justify)
  (save-excursion
    ;; Fill succeeding comment when invoked right before a multi-line
    ;; comment.
    (when (save-excursion
            (beginning-of-line)
            (comment-search-forward (point-at-eol) t))
      (goto-char (match-end 0)))
    (let ((ppss (syntax-ppss))
          (eol (line-end-position)))
      (cond
       ((and (nth 4 ppss)
             (save-excursion
               (goto-char (nth 8 ppss))
               (forward-comment 1)
               (prog1 (not (bolp))
                 (setq eol (point)))))
        ;; Filling inside a comment whose comment-end marker is not \n.
        ;; This code is meant to be generic, so that it works not only for
        ;; css-mode but for all modes.
        (save-restriction
          (narrow-to-region (nth 8 ppss) eol)
          (comment-normalize-vars)      ;Will define comment-continue.
          (let ((fill-paragraph-function nil)
                (paragraph-separate
                 (if (and comment-continue
                          (string-match "[^ \t]" comment-continue))
                     (concat "\\(?:[ \t]*\\(?:"
                             (regexp-quote comment-continue) "\\|"
                             comment-start-skip "\\|"
                             comment-end-skip "\\)\\)?"
                             "\\(?:" paragraph-separate "\\)")
                   paragraph-separate))
                (paragraph-start
                 (if (and comment-continue
                          (string-match "[^ \t]" comment-continue))
                     (concat "\\(?:[ \t]*" (regexp-quote comment-continue)
                             "\\)?\\(?:" paragraph-start "\\)")
                   paragraph-start)))
            (fill-paragraph justify)
            ;; Don't try filling again.
            t)))

       ((and (null (nth 8 ppss))
             (or (nth 1 ppss)
                 (and (ignore-errors
                        (down-list 1)
                        (when (<= (point) eol)
                          (setq ppss (syntax-ppss)))))))
        (goto-char (nth 1 ppss))
        (let ((end (save-excursion
                     (ignore-errors (forward-sexp 1) (copy-marker (point) t)))))
          (when end
            (while (re-search-forward "[{;}]" end t)
              (cond
               ;; This is a false positive inside a string or comment.
               ((nth 8 (syntax-ppss)) nil)
               ;; This is a false positive when encountering an
               ;; interpolated variable (bug#19751).
               ((eq (char-before (- (point) 1)) ?#) nil)
               ((eq (char-before) ?\})
                (save-excursion
                  (forward-char -1)
                  (skip-chars-backward " \t")
                  (when (and (not (bolp))
                             (scss-smie--not-interpolation-p))
                    (newline))))
               (t
                (while
                    (progn
                      (setq eol (line-end-position))
                      (and (forward-comment 1)
                           (> (point) eol)
                           ;; A multi-line comment should be on its own line.
                           (save-excursion (forward-comment -1)
                                           (when (< (point) eol)
                                             (newline)
                                             t)))))
                (if (< (point) eol) (newline)))))
            (goto-char (nth 1 ppss))
            (indent-region (line-beginning-position 2) end)
            ;; Don't use the default filling code.
            t)))))))

(defun css-adaptive-fill ()
  (when (looking-at "[ \t]*/\\*[ \t]*")
    (let ((str (match-string 0)))
      (and (string-match "/\\*" str)
           (replace-match " *" t t str)))))

(defun css-current-defun-name ()
  "Return the name of the CSS section at point, or nil."
  (save-excursion
    (let ((max (max (point-min) (- (point) 1600))))  ; approx 20 lines back
      (when (search-backward "{" max t)
	(skip-chars-backward " \t\r\n")
	(beginning-of-line)
	(if (looking-at "^[ \t]*\\([^{\r\n]*[^ {\t\r\n]\\)")
	    (match-string-no-properties 1))))))

;;; SCSS mode

(defvar scss-mode-syntax-table
  (let ((st (make-syntax-table css-mode-syntax-table)))
    (modify-syntax-entry ?/ ". 124" st)
    (modify-syntax-entry ?\n ">" st)
    ;; Variable names are prefixed by $.
    (modify-syntax-entry ?$ "'" st)
    st))

(defun scss-font-lock-keywords ()
  (append `((,(concat "$" css-ident-re) (0 font-lock-variable-name-face)))
          (css--font-lock-keywords 'sassy)
          `((,(concat "@mixin[ \t]+\\(" css-ident-re "\\)[ \t]*(")
             (1 font-lock-function-name-face)))))

(defun scss-smie--not-interpolation-p ()
  (save-excursion
    (forward-char -1)
    (or (zerop (skip-chars-backward "-[:alnum:]"))
        (not (looking-back "#{\\$" (- (point) 3))))))

;;;###autoload (add-to-list 'auto-mode-alist '("\\.scss\\'" . scss-mode))
;;;###autoload
(define-derived-mode scss-mode css-mode "SCSS"
  "Major mode to edit \"Sassy CSS\" files."
  (setq-local comment-start "// ")
  (setq-local comment-end "")
  (setq-local comment-continue " *")
  (setq-local comment-start-skip "/[*/]+[ \t]*")
  (setq-local comment-end-skip "[ \t]*\\(?:\n\\|\\*+/\\)")
  (setq-local css--at-ids (append css-at-ids scss-at-ids))
  (setq-local css--bang-ids (append css-bang-ids scss-bang-ids))
  (setq-local css--nested-selectors-allowed t)
  (setq-local font-lock-defaults
              (list (scss-font-lock-keywords) nil t)))



(defvar css--mdn-lookup-history nil)

(defcustom css-lookup-url-format
  "https://developer.mozilla.org/en-US/docs/Web/CSS/%s?raw&macros"
  "Format for a URL where CSS documentation can be found.
The format should include a single \"%s\" substitution.
The name of the CSS property, @-id, pseudo-class, or pseudo-element
to look up will be substituted there."
  :version "26.1"
  :type 'string
  :group 'css)

(defun css--mdn-after-render ()
  (setf header-line-format nil)
  (goto-char (point-min))
  (let ((window (get-buffer-window (current-buffer) 'visible)))
    (when window
      (when (re-search-forward "^Summary" nil 'move)
        (beginning-of-line)
        (set-window-start window (point))))))

(defconst css--mdn-symbol-regexp
  (concat "\\("
	  ;; @-ids.
	  "\\(@" (regexp-opt css-at-ids) "\\)"
	  "\\|"
	  ;; ;; Known properties.
	  (regexp-opt css-property-ids t)
	  "\\|"
	  ;; Pseudo-classes.
	  "\\(:" (regexp-opt css-pseudo-class-ids) "\\)"
	  "\\|"
	  ;; Pseudo-elements with either one or two ":"s.
	  "\\(::?" (regexp-opt css-pseudo-element-ids) "\\)"
	  "\\)")
  "Regular expression to match the CSS symbol at point.")

(defconst css--mdn-property-regexp
  (concat "\\_<" (regexp-opt css-property-ids t) "\\s-*\\(?:\\=\\|:\\)")
  "Regular expression to match a CSS property.")

(defconst css--mdn-completion-list
  (nconc
   ;; @-ids.
   (mapcar (lambda (atrule) (concat "@" atrule)) css-at-ids)
   ;; Pseudo-classes.
   (mapcar (lambda (class) (concat ":" class)) css-pseudo-class-ids)
   ;; Pseudo-elements with either one or two ":"s.
   (mapcar (lambda (elt) (concat ":" elt)) css-pseudo-element-ids)
   (mapcar (lambda (elt) (concat "::" elt)) css-pseudo-element-ids)
   ;; Properties.
   css-property-ids)
  "List of all symbols available for lookup via MDN.")

(defun css--mdn-find-symbol ()
  "A helper for `css-lookup-symbol' that finds the symbol at point.
Returns the symbol, a string, or nil if none found."
  (save-excursion
    ;; Skip backward over a word first.
    (skip-chars-backward "-[:alnum:] \t")
    ;; Now skip ":" or "@" to see if it's a pseudo-element or at-id.
    (skip-chars-backward "@:")
    (if (looking-at css--mdn-symbol-regexp)
	(match-string-no-properties 0)
      (let ((bound (save-excursion
		     (beginning-of-line)
		     (point))))
	(when (re-search-backward css--mdn-property-regexp bound t)
	  (match-string-no-properties 1))))))

;;;###autoload
(defun css-lookup-symbol (symbol)
  "Display the CSS documentation for SYMBOL, as found on MDN.
When this command is used interactively, it picks a default
symbol based on the CSS text before point -- either an @-keyword,
a property name, a pseudo-class, or a pseudo-element, depending
on what is seen near point."
  (interactive
   (list
    (let* ((sym (css--mdn-find-symbol))
	   (enable-recursive-minibuffers t)
	   (value (completing-read
		   (if sym
		       (format "Describe CSS symbol (default %s): " sym)
		     "Describe CSS symbol: ")
		   css--mdn-completion-list nil nil nil
		   'css--mdn-lookup-history sym)))
      (if (equal value "") sym value))))
  (when symbol
    ;; If we see a single-colon pseudo-element like ":after", turn it
    ;; into "::after".
    (when (and (eq (aref symbol 0) ?:)
	       (member (substring symbol 1) css-pseudo-element-ids))
      (setq symbol (concat ":" symbol)))
    (let ((url (format css-lookup-url-format symbol))
          (buffer (get-buffer-create "*MDN CSS*")))
      (save-selected-window
        ;; Make sure to display the buffer before calling `eww', as
        ;; that calls `pop-to-buffer-same-window'.
        (switch-to-buffer-other-window buffer)
        (with-current-buffer buffer
          (eww-mode)
          (add-hook 'eww-after-render-hook #'css--mdn-after-render nil t)
          (eww url))))))

(provide 'css-mode)
;;; css-mode.el ends here