[css-contain-3] Add CSSOM API for CSSContainerRule

[lilles]

The spec needs to specify a CSSContainerRule interface for @container rules. It should extend the CSSConditionRule.

[mirisuzanne]

Still fairly new to CSSOM definitions. Can we rely on the attribute CSSOMString conditionText already defined in CSSConditionRule (similar to @supports), or do we need additional attributes here (similar to @media)?

As a start, this is a slightly modified version of what we have for @supports:

The {{CSSContainerRule}} interface represents a ''@container'' rule.

<pre class='idl'>
[Exposed=Window]
interface CSSContainerRule : CSSConditionRule {
};
</pre>

<dl class='idl-attributes'>
  <dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSContainerRule-specific definition for attribute on CSSConditionRule)
  <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
    on getting, must return the condition that was specified,
    without any logical simplifications,
    so that the returned condition will evaluate to the same result
    as the specified condition
    in any conformant implementation of this specification
    (including implementations that implement future extensions
    allowed by the <<general-enclosed>> extensibility mechanism in this specification).
    In other words,
    token stream simplifications are allowed
    (such as reducing whitespace to a single space
    or omitting it in cases where it is known to be optional),
    but logical simplifications (such as removal of unneeded parentheses,
    or simplification based on evaluating results) are not allowed.
</dl>

[tabatkins]

You should be able to rely on it; just copy @supports, yeah.

[mirisuzanne]

Drafted. Agenda+ for a resolution on this approach.

[anttijk]

Please make any attributes readonly.

[lilles]

conditionText is resolved to be readonly in issue #6819, but the spec hasn't been changed yet. I added the NeedsEdits label to the issue.

[astearns]

I think we could resolve on this approach asynchronously. Once the spec has the readonly edit in, I’ll call for objections here and on the mailing list.

[mirisuzanne]

Since conditionText is defined in a separate spec (css-conditional-3), I don't think the readonly change requires any edits on this CSSContainerRule definition?

[lilles]

Since conditionText is defined in a separate spec (css-conditional-3), I don't think the readonly change requires any edits on this CSSContainerRule definition?

No.

[SebastianZ]

Maybe we should also have a getter for the query container. So, depending on what's set for containerName and containerQuery it returns the related element.

Sebastian

[SebastianZ]

In the discussion in #4240, I realized that it may also be useful for authors as well as developer tools to have a getter that exposes the matching query container.

So something like queryContainer that uses the name and condition and spits out the element that currently matches them.

Sebastian