[css-fonts-4] Color fonts are not enabled by default

[litherum]

font-palette indicates that its initial value is normal. The prose for normal says:

User-Agents attempt to treat the color font as closely as possible to a non-color font.

This is kind of unfortunate, as it means that color fonts, by default, will be rendered 'as closely as possible to a non-color font.'

In WebKit, we'll just use the 0th palette by default. I don't know what other browsers do (@drott) but if they do the same then we probably should adjust the description of normal.

[svgeesus]

I agree, the initial value should probably be auto which should mean the UA decides whether to display as color or monochrome based on whatever heuristics they choose. The language about "best legibility" should probably stay.

Implies that we do need another value which means monochrome. And normal does not really convey that.

[drott]

We generally prioritise color tables (COLR (v0 and v1), SBIX, CBDT/CBLC) over contour tables (glyf, CFF2) when present. Thus, if COLR is present and valid, we do not "render it as close as possible to a non-color font" even if it would have glyf glyphs for the same glyph ids. We do use the default palette at index 0 as well, same as WebKit.

In fact, I find the whole paragraph a bit odd. Not sure what its origins are. I haven't seen any implementation synthesising a custom palette for the default rendering.

User-Agents attempt to treat the color font as closely as possible to a non-color font. In particular, user-agents use a palette which will yield the best default result for reading. User-agents may take the computed value of the color property into consideration when making this decision. User agents may also construct and use a palette not defined in the font."

I am in favour of just replacing the description for normal with something like:

User-Agents display the color font with the default palette or default glyph colorisation. User-agents should/must (WDYT?) take the computed value of the color property into consideration when a color font format requires use of the foreground color (for example: color index 0xFFFF in COLR).

Then add a note: "For COLR/CPAL this usually means rendering the font with the palette at index 0."

Regarding:

User agents may also construct and use a palette not defined in the font.

Yes, that's what we do when a @font-palette-values provide custom palette overrides, but I don't think this sentence needs to be in the spec text for normal.

[litherum]

+1 to @drott.

I'm not sure how one would implement a potential monochrome value on Core Text.

[ByteEater-pl]

User-Agents attempt to treat the color font as closely as possible to a non-color font.

That (and some other text there) doesn't look very prescriptive (normative), rather descriptive. Could use some editorial massaging.

[litherum]

Pull request: #7110