Prevent completely tree-shaken styles from triggering excessive CSS

[westonruter]

Summary

This fixes an issue encountered by @delawski where even though a stylesheet may have zero bytes after tree-shaking, it can still end up causing an “Excessive CSS” validation error. The stylesheet here in question is the emoji styles being printed:

amp-wp/includes/class-amp-theme-support.php

Lines 2195 to 2225 in 73cfc46

	/**
	* Print the important emoji-related styles.
	*
	* @see print_emoji_styles()
	* @staticvar bool $printed
	*/
	public static function print_emoji_styles() {
	static $printed = false;
	if ( $printed ) {
	return;
	}
	$printed = true;
	?>
	<style type="text/css">
	img.wp-smiley,
	img.emoji {
	display: inline-block !important; /* Patched from core, which had display:inline */
	border: none !important;
	box-shadow: none !important;
	height: 1em !important;
	width: 1em !important;
	margin: 0 .07em !important;
	vertical-align: -0.1em !important;
	background: none !important;
	padding: 0 !important;
	}
	</style>
	<?php
	}

After quite a bit of trial and error, I got the stylesheets for the theme to add up to the maximum bytes on the page (which is 75000 minus the byte count for the source map comment). I then added a plugin to add a stylesheet before the AMP emoji styles:

<?php
/**
 * Plugin Name: Add Style To Beginning of Head
 */

add_action('wp_head', function () {
	?>
	<style>
		body:after { content:'a' }
	</style>
	<?php
}, 0);

I then created a post which had no emoji on it so that all of the emoji styles would be tree-shaken.

I marked the validation error for the styles from “Add Style To Beginning of Head” as kept to so that those styles would be forcibly added to the page and trigger any other styles as going over the limit.

Before	After

Notice that the entirely tree-shaken stylesheet is no longer showing as excluded and it is no longer raising a validation error.

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for b256089 are ready 🛎️!

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[delawski]

This looks good to me.

When testing locally, I was able to reproduce the issue. After applying this patch, a fully tree shaken stylesheet was no longer marked as "Excluded":

Before	After

[fellyph]

QA Passed