Support completion of classes and IDs in CSS mode

* lisp/textmodes/css-mode.el (css-class-list-function): New variable
holding the function to call for retrieving completions of class
names.
(css-id-list-function): New variable holding the function to call for
retrieving completions of IDs.
(css--foreign-completions): New function for retrieving completions
from other buffers.
(css--complete-selector): Support completing HTML class names and IDs
from other buffers in addition to completing HTML tags.

* lisp/textmodes/sgml-mode.el (html--buffer-classes-cache): New
variable holding a cache for `html-current-buffer-classes'.
(html--buffer-ids-cache): New variable holding a cache for
`html-current-buffer-ids'.
(html-current-buffer-classes): New function returning a list of class
names used in the current buffer.
(html-current-buffer-ids): New function returning a list of IDs used
in the current buffer.
(html-mode): Set `css-class-list-function' and `css-id-list-function'
to `html-current-buffer-classes' and `html-current-buffer-ids'
respectively.

1 files changed, 4 insertions, 2 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 3149d918b56..c3f4cf01b26 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -392,8 +392,10 @@ enables reading of shell initialization files.
 ** CSS mode
 
 ---
-*** Support for completing attribute values, at-rules, bang-rules, and
-HTML tags using the 'completion-at-point' command.
+*** Support for completing attribute values, at-rules, bang-rules,
+HTML tags, classes and IDs using the 'completion-at-point' command.
+Completion candidates for HTML classes and IDs are retrieved from open
+HTML mode buffers.
 
 +++
 ** Emacs now supports character name escape sequences in character and