[css-gaps-1] Define Interpolation behavior for GapDecorations. Issue #12431

[jav099]

This PR aims to define the interpolation behavior for the rule-width and rule-color properties in CSSGapDecorations, based on the resolution in #12431.

Changes can be seen live in https://jav099.github.io/javier-intersections-draft.github.io/

[svgeesus]

@jav099 could you please link your W3C account to your GitHub account, so that the IPR bot knows who you work for? Happy to help if you don't know how to do that.

[jav099]

@svgeesus I have already done so I think? The IPR bot was failing yesterday and I followed its links to link the accounts. Now if I go to my W3C account and look at my connected accounts I can see my github profile.

[kbabbitt]

Suggested change

	Animation type: repeatable list, (see [[#repeat-interpolation]]).
	Animation type: repeatable list, see [[#repeat-interpolation]].

[kbabbitt]

Suggested change

	Animation type: repeatable list, (see [[#repeat-interpolation]]).
	Animation type: repeatable list, see [[#repeat-interpolation]].

[kbabbitt]

Half sentence here (left over from drafting?)

[kbabbitt]

Suggested change

	<li> <dfn>Expansion</dfn>: Expand each ''repeat(n, …)'' into its equivalent list of values. </li>
	<li> <dfn>Expansion</dfn>: Expand each [=integer repeater=] into its equivalent list of values. </li>

(link to defined terms where available for consistency)

[kbabbitt]

Suggestions for the interpolation examples:

• Applying the example class to <pre> makes the formatting inconsistent with other examples in the document; apply it to <div> instead and perhaps include some explanatory text
• I think it would be helpful to include the expanded from and to lists also
• Adjust the spacing so that corresponding values line up vertically in the formatted HTML (unfortunately, as best I know, this can take some trial-and-error)

Here's how I might mark one of these up, but feel free to vary the overall form and explanatory text between examples as it makes sense to do so:

	<div class="example">
		<pre class="lang-css">
		@keyframes example {
			from { column-rule-width: repeat(2, 10px 20px); }
			to   { column-rule-width: 20px; }
		}
		</pre>
		Interpolation of the above values would result in expansion of the
		"from" and "to" values to create lists of equal lengths:
		<pre>
		From:   10px 20px 10px 20px
		At 50%: 15px 20px 15px 20px
		To:     20px 20px 20px 20px
		</pre>
	</div>

(I just now discovered class="lang-css" while looking at other specs; I'll probably go back and mark up the other examples in this spec with it at some point)

[kbabbitt]

Suggested change

	When either of the lists we are interpolating between (let's call them |from| and |to|) include an <a>auto repeater</a> (e.g. ''repeat(auto, …)''), we conceptually split the list into three segments, similar
	When either of the lists we are interpolating between (let's call them |from| and |to|) include an <a>auto repeater</a>, we conceptually split the list into three segments, similar

(the "e.g." is not needed since the autolink will take the reader to the definition of the term; omit it for brevity)

[kbabbitt]

I think these rules also apply to cases where the "from" and "to" values don't contain any repeaters but do have unequal list lengths, correct? Assuming that's the case, I'd suggest generalizing the section title and introductory test to reflect that.

[kbabbitt]

Are we interpolating part of the value continuously and part discretely? That seems weird to me - I'd expect that if the lists can't be lined up, the entire interpolation would be treated as discrete. Though maybe there's precedent for doing it this way elsewhere?

[kbabbitt]

For cases where a discrete interpolation happens, the "At 50%" line doesn't add much value. I would instead have prose stating that the interpolation is discrete and explaining the specific reason why (e.g. "because only one value contains an [=auto repeater=]").

[kbabbitt]

Suggested change

	<li>Specified the interpolation behavior for ''repeat()'', 'rule-color', and 'rule-width'.
	<li>Specified the interpolation behavior for values which may contain repeaters.