[csswg-drafts] [css-regions] Region cannot be used as an IDL type (#5519)

foolip has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-regions] Region cannot be used as an IDL type ==
https://drafts.csswg.org/css-regions/#idl-index current has this:
```
partial interface Document {
  readonly attribute NamedFlowMap namedFlows;
};

[Exposed=Window,
 MapClass=(CSSOMString, NamedFlow)] interface NamedFlowMap {
  NamedFlow? get(CSSOMString flowName);
  boolean has(CSSOMString flowName);
  NamedFlowMap set(CSSOMString flowName, NamedFlow flowValue);
  boolean delete(CSSOMString flowName);
};

[Exposed=Window]
interface NamedFlow : EventTarget {
  readonly attribute CSSOMString name;
  readonly attribute boolean overset;
  sequence<Region> getRegions();
  readonly attribute short firstEmptyRegionIndex;
  sequence<Node> getContent();
  sequence<Region> getRegionsByContent(Node node);
};

interface mixin Region {
  readonly attribute CSSOMString regionOverset;
  sequence<Range>? getRegionFlowRanges();
};

Element includes Region;
```

`Region` is used as a type in the `sequence<Region>` return value. However, `Region` isn't an [IDL type](https://heycam.github.io/webidl/#idl-types), it's a piece of syntax like partial interfaces. My best guess/understanding is that it's not possible to use interface mixins as a shorthand for the interfaces that include those mixins.

So, probably `sequence<Element>` should probably be used here instead.

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5519 using your GitHub account


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