[css-color] Add note for relationship between color property and color glyphs

[upsuper]

Per discussion in the mailing list.

[Crissov]

Text color (and its opacity) should probably not affect PNGinOTF glyphs (CBDT and Apple), but should it also not affect fonts with COLR or SVG  tables? Should we solely rely on future explicit CPAL control?

#352 is somewhat related and I hope to see the issue on a WG agenda soon.

[svgeesus]

The specification for COLR and CPAL says that the color values are in sRGB.
The specification for SVG allows whatever CSS defines.

CSS4 Fonts does, as @Crissov mentions, allow palette selection, and new palettes in CSS.

[svgeesus]

If the color glyph uses CPAL then there is a special value for currentColor so this note is not strictly true. Also for SVGinOpenType glyphs, currentColor could be used s the note is incorrect in that case also.

[tabatkins]

I'd prefer text more like:

Note: This property has no effect on "color glyphs", such as emoji in some fonts, which are explicitly colored by a built-in palette, unless explicitly specified otherwise by a font specification. For example, some colored fonts use a palette with the ability to refer to the "current color", such as the CPAL table in OpenType; in that case the "current color" is set by this property, identically to how the ''currentcolor'' value works.

Am I referring to things correctly here? I'm just assuming CPAL is an OpenType table.

[litherum]

I prefer @tabatkins's comment too, because it is more descriptive.

[svgeesus]

Am I referring to things correctly here? I'm just assuming CPAL is an OpenType table.

It is, yes.
https://www.microsoft.com/typography/otspec/cpal.htm