Add command for cycling between CSS color formats

* lisp/textmodes/css-mode.el (css-mode-map): Add keybinding for
'css-cycle-color-format'.
(css--rgb-color): Add support for extracting alpha component.
(css--hex-alpha, css--color-to-4-dpc, css--named-color-to-hex)
(css--format-rgba-alpha, css--hex-to-rgb)
(css--rgb-to-named-color-or-hex): New functions.
(css-cycle-color-format): New command for cycling between color
formats.

* test/lisp/textmodes/css-mode-tests.el (css-test-color-to-4-dpc):
(css-test-named-color-to-hex, css-test-format-rgba-alpha)
(css-test-hex-to-rgb, css-test-rgb-to-named-color-or-hex)
(css-test-cycle-color-format, css-test-hex-alpha): New tests for the
changes mentioned above.

* etc/NEWS: Mention the new command.

1 files changed, 7 insertions, 0 deletions

diff --git a/etc/NEWS b/etc/NEWS
index bec7753d192..1382f96a374 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -77,6 +77,13 @@ whether '"' is also replaced in 'electric-quote-mode'.  If non-nil,
 
 * Changes in Specialized Modes and Packages in Emacs 27.1
 
+** CSS mode
+
+---
+*** A new command 'css-cycle-color-format' for cycling between color
+formats (e.g. "black" => "#000000" => "rgb(0, 0, 0)") has been added,
+bound to 'C-c C-f'.
+
 ** Dired
 
 +++