add mhtml-mode.el

* etc/NEWS: Update.
* lisp/textmodes/mhtml-mode.el: New file.
* test/manual/indent/html-multi.html: New file.
* test/lisp/textmodes/mhtml-mode-tests.el: New file.
* doc/emacs/text.texi (HTML Mode): Mention mhtml-mode.

Author: tromey

doc/emacs/text.texi

@@ -1899,8 +1899,14 @@ between Latin-1 encoded files and @TeX{}-encoded equivalents.
 @findex html-mode
 
   The major modes for SGML and HTML provide indentation support and
-commands for operating on tags.  HTML mode is a slightly customized
-variant of SGML mode.
+commands for operating on tags.
+
+  HTML consists of two modes---one, a basic mode called
+@code{html-mode} is a slightly customized variant of SGML mode.  The
+other, which is used by default for HTML files, is called
+@code{mhtml-mode}, and attempts to properly handle Javascript enclosed
+in a @code{<script>} element and CSS embedded in a @code{<style>}
+element.
 
 @table @kbd
 @item C-c C-n

etc/NEWS

@@ -753,6 +753,13 @@ processes on exit.
 
 ** New library 'xdg' with utilities for some XDG standards and specs.
 
+** HTML
+
++++
+*** A new submode of `html-mode', `mthml-mode', is now the default
+    mode for *.html files.  This mode handles indentation,
+    fontification, and commenting for embedded JavaScript and CSS.
+
 
 * Incompatible Lisp Changes in Emacs 26.1