Auto merge of #29711 - switchpiggy:sp3, r=mrobinson

Implemented special cases for Element.clientWidth() and clientHeight()

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Implemented the special cases for Element.clientWidth() and Element.clientHeight() as outlined in issue #29704.

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- [x] `./mach test-tidy` does not report any errors

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[Specification text](https://w3c.github.io/csswg-drafts/cssom-view/#dom-element-clientwidth):
> If the element is the [root element](https://w3c.github.io/csswg-drafts/css-display-4/#root-element) and the element’s [node document](https://dom.spec.whatwg.org/#concept-node-document) is not in [quirks mode](https://dom.spec.whatwg.org/#concept-document-quirks), or if the element is [the body element](https://html.spec.whatwg.org/multipage/dom.html#the-body-element-2) and the element’s node document is in quirks mode, return the [viewport](https://www.w3.org/TR/CSS21/visuren.html#x1) width excluding the size of a rendered scroll bar (if any).

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