Provide ability to bypass conversion of inline style attributes and !important qualifiers for AMP-first CSS

[westonruter]

Feature description

As reported in GoogleForCreators/web-stories-wp#6783, the style sanitizer's conversion of inline style attributes (and !important qualifiers) can result in ballooning the size of style[amp-custom]. I commented in GoogleForCreators/web-stories-wp#6783 (comment):

Since the AMP plugin primarily has been focused on converting vanilla HTML into valid AMP, it has to process anything that a theme/plugin may throw at it. Originally AMP didn't allow inline style attributes, so that is why initially we moved inline styles to style[amp-custom]. Nevertheless, there is another key reason to move the styles: themes/plugins may add inline styles which also contain !important qualifiers, and !important is not yet allowed by AMP. In order to preserve the CSS cascade, we convert inline styles to style rules with high-specificity CSS selectors using the :not(#_) hack. When !important is present, we multiply the factor of the number of :not(#_) selectors to ensure they are original cascade is preserved. You can see some more exploration of this in #1313.

This all being said, it seems useful to have a way to configure the style sanitizer to bypass the transformation of style attributes into style[amp-custom] while also bypassing the !important property transformations. With these conversions bypassed, the sanitizer would have to strip out the !important qualifiers in order to maintain AMP validity. This seems reasonable in the case of Web Stories since you have complete control over the page construction as being AMP-first.

Having this capability would be useful for any site that is intentionally writing CSS in an AMP-first way.

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

This is less important now due to a better solution for Web Stories: GoogleForCreators/web-stories-wp#6783 (comment).

[westonruter]

Maybe later.

[westonruter]

Note that this did end up getting implemented, but it requires that you are not targeting for a valid AMP page. See #6589.