bug#23383: [PATCH] Support completion of color functions in CSS mode

[Simen Heggestøyl]

Thanks for the revision, Etienne.

There are still a couple of issues that need to be resolved.

- I think we should stick to the current CSS Color Module Level 3

  recommendation [1]. The functions apart from `rgb()', `rgba()',

  `hsl()', and `hsla()' are only present in an Editor's Draft. I find no

  other resources about them, so I don't think we should include them

  until the new spec becomes at least a Candidate Recommendation.

- I think there's little use in completing `#' since it's only one

  character.

- `namedColor' must be changed to `named-color' to match the name of the

  value class.

- The new color names need to be double-checked. For instance I see that

  `limegreen' has become `limeg een' and `salmon' has become `sa mon'.

- The test `css-test-property-values' in

  test/lisp/textmodes/css-mode-tests.el needs a small update.

[1] https://www.w3.org/TR/css3-color/

-- Simen