[css-pseudo] Extend CSSPseudoElement to all standardized non-element-backed tree-abiding pseudo-elements

[danielsakhapov]

From #13804

[Copilot]

Pull request overview

Extends the CSSPseudoElement interface's allowed type values from the previous fixed list (::before, ::after, ::marker) to all standardized tree-abiding pseudo-elements that are not element-backed, adding ::backdrop and ::view-transition as illustrative examples, and updates the Element.pseudo() algorithm to return null when the parsed selector refers to an element-backed pseudo-element.

Changes:

• Reworded the description of CSSPseudoElement.type to reference the [=tree-abiding=] / [=element-backed=] terminology and converted the value list into examples.
• Added ::backdrop and ::view-transition entries to the example <dl>.
• Added a new step in the pseudo() algorithm that returns null for element-backed pseudo-elements.

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