Thanks again for the feedback, Stefan! An updated patch follows.

-- Simen


 From 839810c2fc79d64634be2d27148df9f0759e1c8b Mon Sep 17 00:00:00 2001
 From: =?UTF-8?q?Simen=20Heggest=C3=B8yl?= <simenheg@gmail.com>
Date: Thu, 9 Apr 2015 19:09:04 +0200
Subject: [PATCH] css-mode.el: Support multi-line comment filling

Fixes: debbugs:20256

* lisp/textmodes/css-mode.el (css-fill-paragraph): Support multi-line
comment filling.
(css-adaptive-fill): New function.
(css-mode): Set `adaptive-fill-function'.
(scss-mode): Set `comment-continue'.
---
 lisp/textmodes/css-mode.el | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/lisp/textmodes/css-mode.el b/lisp/textmodes/css-mode.el
index d1893a3..424cdb7 100644
--- a/lisp/textmodes/css-mode.el
+++ b/lisp/textmodes/css-mode.el
@@ -381,7 +381,8 @@ pseudo-classes, and at-rules."
   (setq-local comment-start-skip "/\\*+[ \t]*")
   (setq-local comment-end "*/")
   (setq-local comment-end-skip "[ \t]*\\*+/")
-  (setq-local fill-paragraph-function 'css-fill-paragraph)
+  (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
@@ -395,6 +396,12 @@ pseudo-classes, and at-rules."

 (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
@@ -414,8 +421,11 @@ pseudo-classes, and at-rules."
                 (paragraph-separate
                  (if (and comment-continue
                           (string-match "[^ \t]" comment-continue))
-                     (concat "\\(?:[ \t]*" (regexp-quote 
comment-continue)
-                             "\\)?\\(?:" paragraph-separate "\\)")
+                     (concat "\\(?:[ \t]*\\(?:"
+                             (regexp-quote comment-continue) "\\|"
+                             comment-start-skip "\\|"
+                             comment-end-skip "\\)\\)?"
+                             "\\(?:" paragraph-separate "\\)")
                    paragraph-separate))
                 (paragraph-start
                  (if (and comment-continue
@@ -468,6 +478,12 @@ pseudo-classes, and at-rules."
             ;; 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
@@ -504,6 +520,7 @@ pseudo-classes, and at-rules."
   "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 font-lock-defaults '(scss-font-lock-keywords nil t)))
-- 
2.1.4