Error while opening an HTML file with Emacs 26.1

[Pascal Quesseveur <pquessev@gmail.com>]

Emacs 26.1 on Windows, 32 bits distrib

Using 'emacs -Q' I get that error when opening an HTML file:

Debugger entered--Lisp error: (file-missing "Cannot open load file"
"No such file or directory" "css-mode")

  require(css-mode)
  byte-code("..." [require flyspell sgml-mode js css-mode prog-mode font-lock custom-declare-variable mhtml-tag-relative-indent funcall function #f(compiled-function () #<bytecode 0x33759f9>) "How <script> and <style> bodies are indented relative to the tag.\n\nWhen t, indentation looks like:\n\n  <script>\n    code();\n  </script>\n\nWhen nil, indentation of the script body starts just below the\ntag, like:\n\n  <script>\n  code();\n  </script>\n\nWhen `ignore', the script body starts in the first column, like:\n\n  <script>\ncode();\n  </script>" :group sgml :type (choice (const nil) (const t) (const ignore)) :safe symbolp :version "26.1"] 12)
  mhtml-mode()
  set-auto-mode-0(mhtml-mode nil)
  set-auto-mode()
  normal-mode(t)
  after-find-file(nil t)
  find-file-noselect-1(#<buffer simple-file.html> "~/dev/simple-file.html" nil nil "~/dev/simple-file.html" ((4608 2 . 8750) (20619 . 8025)))
  find-file-noselect("C:/Users/pascalq/dev/simple-file.html" nil nil nil)
  find-file("C:/Users/pascalq/dev/simple-file.html")
  dnd-open-local-file("file:C%3A/Users/pascalq/dev/simple-file.html" private)
  dnd-handle-one-url(#<window 3 on *scratch*> private "file:C%3A/Users/pascalq/dev/simple-file.html")
  w32-handle-dropped-file(#<window 3 on *scratch*> "C:\\Users\\pascalq\\dev\\simple-file.html")
  apply(w32-handle-dropped-file (#<window 3 on *scratch*> "C:\\Users\\pascalq\\dev\\simple-file.html"))
  #f(compiled-function (&rest args2) #<bytecode 0x2a7261b>)("C:\\Users\\pascalq\\dev\\simple-file.html")
  mapc(#f(compiled-function (&rest args2) #<bytecode 0x2a7261b>) ("C:\\Users\\pascalq\\dev\\simple-file.html"))
  w32-drag-n-drop((drag-n-drop (#<window 3 on *scratch*> 146 (405 . 337) -521301871 nil 146 (40 . 3) nil (405 . 277) (10 . 20)) ("C:\\Users\\pascalq\\dev\\simple-file.html")))
  funcall-interactively(w32-drag-n-drop (drag-n-drop (#<window 3 on *scratch*> 146 (405 . 337) -521301871 nil 146 (40 . 3) nil (405 . 277) (10 . 20)) ("C:\\Users\\pascalq\\dev\\simple-file.html")))
  call-interactively(w32-drag-n-drop nil nil)
  command-execute(w32-drag-n-drop)

-- 
Pascal Quesseveur
pquessev@gmail.com