CSS3 colors seem to be compatible with all Grade C+ browsers according to https://caniuse.com/css3-colors, so there is no blocker on this?

Alpha hexademical notation has a different support status: https://caniuse.com/mdn-css_types_color_alpha_hexadecimal_notation

I have searched for more listed features:

• types: <color>: RGB functional notation (rgb()) https://caniuse.com/mdn-css_types_color_rgb_functional_notation
• types: <color>: rgb() can accept alpha values https://caniuse.com/mdn-css_types_color_rgb_function_accepts_alpha → Bad
• types: <color>: Allow floats in rgb() and rgba() https://caniuse.com/mdn-css_types_color_floats_in_rgb_rgba → Bad
• types: <color>: HSL color values (hsl()) https://caniuse.com/mdn-css_types_color_hsl
• types: <color>: Alpha color values (rgba(), hsla()) https://caniuse.com/mdn-css_types_color_alpha
• #rrggbbaa hex color notation (UNOFF) https://caniuse.com/css-rrggbbaa → Bad

rgb(), hsl(), rgba() and hsla() seem to be compatible with all Grade C+ browsers except Android 4.3+, which is noted as 'support unknown'.
Anyway, if #rrggaabb is unofficial, does it mean this task should be declined? Or be retitled to support other opacity features?

Anyway, if #rrggaabb is unofficial, does it mean this task should be declined?

I believe you just found a bug in caniuse. Could you file an issue there and cc me?

I think I can't because I am not sure about #rrggbbaa. it is not found on https://www.w3.org/TR/css-color-3/ but described on mdn web docs.

It is on css-color-4 and I uploaded a PR https://github.com/Fyrd/caniuse/pull/6240

Change #1152895 had a related patch set uploaded (by Alistair3149; author: Alistair3149):

[css-sanitizer@master] Add support for alpha value in hex color notation

https://gerrit.wikimedia.org/r/1152895

Change #1152895 merged by jenkins-bot:

[css-sanitizer@master] Update Color to Level 4

https://gerrit.wikimedia.org/r/1152895

In T265675#11067084, @Stang wrote:

4 or 8-digit hex colors still not supported. However, I can use rgba() as a workaround.

The css-sanitizer has been updated to support this syntax, but TemplateStyles in REL1_XX branches does not require the updated css-sanitizer library, only in master branch.