Opaque colours serialize as rgba(r, g, b, 1) where CSSOM specifies rgb(r, g, b)

[lahma]

Observed while generating JavaScript bindings for AngleSharp's CSSOM in Jint (sebastienros/jint#3634), on the current AngleSharp.Css.

A colour value serializes as rgba(r, g, b, 1) for a fully opaque colour, where CSSOM's serialization of a <color> (https://drafts.csswg.org/cssom/#serialize-a-css-component-value, the rgb() / rgba() rule: "if the alpha is 1, serialize as rgb()") gives rgb(r, g, b). Scripts and tests that compare element.style.color against rgb(255, 0, 0) see the difference; browsers all serialize the opaque case as rgb(). Happy to send a PR if you would take one.

[FlorianRappl]

This is something I am aware of, but the serialization choice was done so on purpose.

I'd actually leave it with the current behavior as its easier to work with from a .NET perspective and should not break anything in JS land etc. (one would always need to expect / have rgba implemented there, too).

[FlorianRappl]

What we could do is introduce a new static property (much like UseHex, see https://github.com/AngleSharp/AngleSharp.Css/blob/devel/src/AngleSharp.Css/Values/Primitives/CssColorValue.cs#L470), e.g., UseSpecSerialization (better name yet to be found). This would be opt-in (i.e., default= false) to be non-breaking.

If active we'd use the improved serialization skipping alpha values in case there is no transparency. To be fully spec-compliant we'd potentially also need to extend the color value with information if it was derived from a fixed (named) color - this way reducing the output to the used name.

[lahma]

Understood, and the opt-in is the right shape. PR coming for a static switch next to UseHex that drops the alpha when it is 1; the named-colour reduction can be a second step.

[lahma]

PR: #229 — UseSpecSerialization next to UseHex, default off; opaque colours serialize as rgb() when on. Kept your placeholder name so the named-colour reduction can sit behind the same switch later; OmitOpaqueAlpha is the alternative if you prefer the name to describe only today's behaviour.