Add spec text for arraylike<CSSTransformComponent> in CSSTransformValue

[nainar]

this is blocked by: whatwg/webidl#345

[css-meeting-bot]

The Working Group just discussed Add spec text for arraylike in CSSTransformValue, and agreed to the following resolutions:

• RESOLVED: use indexed getters/setter - and then bring this up at TC39 meeting to just indexed property interception

The full IRC log of that discussion

<nainar> Topic: Add spec text for arraylike in CSSTransformValue
<dholbert> github issue: https://github.com//issues/487
<nainar> TabAtkins: have arraylike eg transformvalue, unparsed value. There will be more.
<nainar> TabAtkins: Probem is WebIDL doesnt allow us to do typechecking on array.
<nainar> TabAtkins: So working around is indexed accessors that invokes proxy machinery. Only way to not do expplicit type check
<nainar> TabAtkins: It would also be annoyign to implemnt/spec up
<nainar> TabAtkins: Propose to use indexed getters/setter - and then bring this up at TC39 meeting. Proposal to allow cheap accessto indexed props.
<nainar> TabAtkins: Good discussion 10 years ago. Support from brosers at tht time.
<nainar> TabAtkins: We can switch over to that - cheaper than full proxy.
<nainar> TabAtkins: Proposal is to start with indexed accessors, and then switch to just indexed propoerty interception when Tab propses it to TC39
<nainar> TabAtkins: We then have good early type error for things like transform values. We can throw on appending.
<nainar> TabAtkins: Also less typechecking logic in spec text . But proxies are more expensive. But we already use this in a few places in teh paltform.
<nainar> Rossen: Seems reasonable. How likely TC39 will adopt?
<nainar> TabAtkins: Dont know about when. But people supported it then and they qre still round. related proposals keep coming up.
<nainar> TabAtkins: So something related would get accepted easily.
<nainar> RESOLVED: use indexed getters/setter - and then bring this up at TC39 meeting to just indexed property interception

[nainar]

Marking this as Needs-Edits for the first action