Eliminate obsolete transformation of @-moz-document url-prefix() Firefox CSS hack into @supports (-moz-appearance:meterbar)

[westonruter]

Feature description

The @-moz-document CSS at-rule is now allowed in AMP as of ampproject/amphtml#26406 so we no longer need to transform this Firefox-specific CSS hack into another Firefox CSS hack, which was implemented in #5530.

Specifically, this code is no longer necessary:

amp-wp/includes/sanitizers/class-amp-style-sanitizer.php

Lines 2155 to 2171 in a5d5237

	if (
	'-moz-document' === $css_item->atRuleName()
	&&
	'url-prefix()' === $css_item->atRuleArgs()
	&&
	in_array( 'supports', $options['allowed_at_rules'], true )
	) {
	// Replace `@-moz-document url-prefix()` with `@supports (-moz-appearance:meterbar)` as an alternative
	// way to provide Firefox-specific style rules. This is a workaround since @-moz-document is not
	// yet allowed in AMP, and this use of @supports is another recognized Firefox-specific CSS hack,
	// per <http://browserhacks.com/#hack-8e9b5504d9fda44ec75169381b3c3157>.
	// For adding @-moz-document to AMP, see <https://github.com/ampproject/amphtml/issues/26406>.
	$new_css_item = new AtRuleBlockList( 'supports', '(-moz-appearance:meterbar)' );
	$new_css_item->setContents( $css_item->getContents() );
	$this->replace_inside_css_list( $css_list, $css_item, [ $new_css_item ] );
	$css_item = $new_css_item; // To process_css_list below.
	$sanitized = false;

---

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Acceptance criteria

Implementation brief

QA testing instructions

Demo

Changelog entry

[westonruter]

QA Passed

When switching to Twenty Nineteen and I add a block with a drop cap:

<!-- wp:paragraph {"dropCap":true} -->
<p class="has-drop-cap">This has a drop cap!</p>
<!-- /wp:paragraph -->

On an AMP page I can see in the source:

@-moz-document url-prefix(){.entry .entry-content .has-drop-cap:not(:focus):first-letter{margin-top:.2em}}

And the page is valid AMP.