Use the CSS convention for #RGB colors (bug#36304)

* src/xterm.c (x_parse_color): Change interpretation of #RGB color
triplets to match CSS rather than X conventions.

* lisp/term/tty-colors.el (tty-color-standard-values): Change
interpretation of #RGB color triplets to match CSS rather than X
conventions.  Allow upper-case digits.  Fix rgb:R/G/B
interpretation.

* doc/emacs/display.texi (Colors): Specify the convention used for
"#RGB" color triplets.

* test/lisp/tty-colors-tests.el: New file.

* etc/NEWS: Mention the change.

1 files changed, 6 insertions, 1 deletions

diff --git a/etc/NEWS b/etc/NEWS
index 021e84c9a43..c970a3c55d1 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -407,6 +407,12 @@ names in xref buffers.
 
 ** New variable `file-size-function' controls how file sizes are displayed.
 
++++
+** Emacs now interprets RGB triplets like HTML, SVG, and CSS do.
+
+The X convention previously used differed slightly, particularly for
+RGB triplets with a single hexadecimal digit per component.
+
 
 * Editing Changes in Emacs 27.1
 
@@ -1509,7 +1515,6 @@ automatically updates.  In the buffer, you can use 's q' or 's e' to
 signal a thread with quit or error respectively, or get a snapshot
 backtrace with 'b'.
 
-
 ** thingatpt.el
 
 ---