* lisp/textmodes/css-mode.el (css--format-hex): New function for
shortening CSS hex colors when possible.
(css--named-color-to-hex, css--rgb-to-named-color-or-hex): Use it.
* test/lisp/textmodes/css-mode-tests.el (css-test-format-hex): New
tests for 'css--format-hex'.
(css-test-named-color-to-hex, css-test-cycle-color-format): Adjust for
the changes to 'css--named-color-to-hex' and
'css--rgb-to-named-color-or-hex'.
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