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.

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

lisp/textmodes/css-mode.el

@@ -30,7 +30,6 @@
 ;; - electric ; and }
 ;; - filling code with auto-fill-mode
 ;; - fix font-lock errors with multi-line selectors
-;; - support completion of user-defined classes names and IDs
 
 ;;; Code:
 
@@ -864,16 +863,46 @@ Used to provide completion of HTML tags in selectors.")
   "Non-nil if nested selectors are allowed in the current mode.")
 (make-variable-buffer-local 'css--nested-selectors-allowed)
 
-;; TODO: Currently only supports completion of HTML tags.  By looking
-;; at open HTML mode buffers we should be able to provide completion
-;; of user-defined classes and IDs too.
+(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)
-    (save-excursion
-      (let ((end (point)))
+    (let ((end (point)))
+      (save-excursion
         (skip-chars-backward "-[:alnum:]")
-        (list (point) end css--html-tags)))))
+        (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.

lisp/textmodes/sgml-mode.el

@@ -32,6 +32,9 @@
 
 ;;; Code:
 
+(require 'dom)
+(require 'seq)
+(require 'subr-x)
 (eval-when-compile
   (require 'skeleton)
   (require 'cl-lib))
@@ -2168,6 +2171,55 @@ This takes effect when first loading the library.")
 	 nil t)
 	(match-string-no-properties 1))))
 
+(defvar html--buffer-classes-cache nil
+  "Cache for `html-current-buffer-classes'.
+When set, this should be a cons cell where the CAR is the
+buffer's tick counter (as produced by `buffer-modified-tick'),
+and the CDR is the list of class names found in the buffer.")
+(make-variable-buffer-local 'html--buffer-classes-cache)
+
+(defvar html--buffer-ids-cache nil
+  "Cache for `html-current-buffer-ids'.
+When set, this should be a cons cell where the CAR is the
+buffer's tick counter (as produced by `buffer-modified-tick'),
+and the CDR is the list of class names found in the buffer.")
+(make-variable-buffer-local 'html--buffer-ids-cache)
+
+(defun html-current-buffer-classes ()
+  "Return a list of class names used in the current buffer.
+The result is cached in `html--buffer-classes-cache'."
+  (let ((tick (buffer-modified-tick)))
+    (if (eq (car html--buffer-classes-cache) tick)
+        (cdr html--buffer-classes-cache)
+      (let* ((dom (libxml-parse-html-region (point-min) (point-max)))
+             (classes
+              (seq-mapcat
+               (lambda (el)
+                 (when-let (class-list
+                            (cdr (assq 'class (dom-attributes el))))
+                   (split-string class-list)))
+               (dom-by-class dom ""))))
+        (setq-local html--buffer-classes-cache (cons tick classes))
+        classes))))
+
+(defun html-current-buffer-ids ()
+  "Return a list of IDs used in the current buffer.
+The result is cached in `html--buffer-ids-cache'."
+  (let ((tick (buffer-modified-tick)))
+    (if (eq (car html--buffer-ids-cache) tick)
+        (cdr html--buffer-ids-cache)
+      (let* ((dom
+              (libxml-parse-html-region (point-min) (point-max)))
+             (ids
+              (seq-mapcat
+               (lambda (el)
+                 (when-let (id-list
+                            (cdr (assq 'id (dom-attributes el))))
+                   (split-string id-list)))
+               (dom-by-id dom ""))))
+        (setq-local html--buffer-ids-cache (cons tick ids))
+        ids))))
+
 
 ;;;###autoload
 (define-derived-mode html-mode sgml-mode '(sgml-xml-mode "XHTML" "HTML")
@@ -2218,6 +2270,12 @@ To work around that, do:
   (setq-local add-log-current-defun-function #'html-current-defun-name)
   (setq-local sentence-end-base "[.?!][]\"'”)}]*\\(<[^>]*>\\)*")
 
+  (when (fboundp 'libxml-parse-html-region)
+    (defvar css-class-list-function)
+    (setq-local css-class-list-function #'html-current-buffer-classes)
+    (defvar css-id-list-function)
+    (setq-local css-id-list-function #'html-current-buffer-ids))
+
   (setq imenu-create-index-function 'html-imenu-index)
 
   (setq-local sgml-empty-tags