[css-text-decor-3] Clarify expected computed value for `text-decoration` and similar shorthands (can we omit resolved `currentColor`?)

[vitorroriz]

As of right now, text-decoration computed WPT expects text-decoration: none to be serialized to the resolved value of currentColor.

However, since currentColor is the default value of text-decoration-color, one might expect that we should omit it and the computed value should be none, following canonical order and short serialization rule, since none is the default value for text-decoration-line, which is the first longhand in canonical order.

For another similar shorthand property, text-emphasis that also has as a value of one of its longhands, It seems that for text-emphasis computed WPT, Safari, Chrome and Firefox agree that text-emphasis: none should be serialized to none <resolved-value-for-current-color>

[nt1m]

I think none is the correct serialization here, because it's the most backwards compatible serialization (the only one which will work in older Safari when round-tripping). This is the number 1 criteria, followed by shortest serialization in CSSOM.

My proposal for text-decoration shorthand serialization:

1. if text-decoration-line is none, return none
2. otherwise, return shortest possible serialization in canonical order

[fantasai]

Agree with @nt1m's logic.

It does open the question of whether we should make a general rule that a computed value of currentColor serializes out as nothing rather than as the resolved value when it's an otherwise omittable value?

[nt1m]

My logic more revolves around the other values being meaningless when text-decoration-line is set to none, so text-decoration: none green would serialize to text-decoration: none according to my logic.

[fantasai]

@nt1m No, that definitely doesn't work. The computed value of none green and none are not the same. As an example, if you round-tripped it and then added a text-decoration-line: underline, that would break if you serialized none green as none.

[Loirooriol]

text-decoration is one of the cases discussed in #8155

The proposal there was to choose the 1st option in grammar order. https://drafts.csswg.org/css-text-decor-3/#text-decoration-property defines the grammar as

<'text-decoration-line'> || <'text-decoration-style'> || <'text-decoration-color'>

so the serialization would be the <'text-decoration-line'> component, i.e. none.

[vitorroriz]

So, like this?
WebKit/WebKit#48273