background-image-svg-data-url.css

#try-stylesheet-containing-closing-style-tag {
	background-color: red;
}

#try-stylesheet-containing-closing-style-tag {
	background-image: url( "data:image/svg+xml;utf8,<svg xmlns='http://www.w3.org/2000/svg' version='1.1' viewBox='0 0 30 30' width='30' height='30'><title>AMP</title><defs><style>.circle{fill:white}.overlay{fill:blue;}</style></defs><g fill='none' fill-rule='evenodd'><path class='circle' d='M0 15c0 8.284 6.716 15 15 15 8.285 0 15-6.716 15-15 0-8.284-6.715-15-15-15C6.716 0 0 6.716 0 15z'/><path class='overlay' fill-rule='nonzero' d='M13.85 24.098h-1.14l1.128-6.823-3.49.005h-.05a.57.57 0 0 1-.568-.569c0-.135.125-.363.125-.363l6.272-10.46 1.16.005-1.156 6.834 3.508-.004h.056c.314 0 .569.254.569.568 0 .128-.05.24-.121.335L13.85 24.098zM15 0C6.716 0 0 6.716 0 15c0 8.284 6.716 15 15 15 8.285 0 15-6.716 15-15 0-8.284-6.715-15-15-15z'/></g></svg>" );
	background-repeat: no-repeat;
	background-color: lime;
	background-position: bottom left;
	background-size: 50px 50px;
	height: 100px;
}

content-property.css

#try-stylesheet-containing-closing-style-tag::after {
	content: " FAIL!";
	color: red;
}

#try-stylesheet-containing-closing-style-tag::after {
	content: " PASS! </style>";
	color: green;
}

try-stylesheet-containing-closing-style-tag.php

<?php
/**
 * Try Stylesheet Containing Closing Style Tag plugin bootstrap.
 *
 * @package   Google\Try_Stylesheet_Containing_Closing_Style_Tags
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2021 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: Try Stylesheet Containing Closing Style Tags
 * Plugin URI: https://gist.github.com/westonruter/35959595840823f77e4f85e584578bc3
 * Description: Test case for <a href="https://github.com/ampproject/amp-wp/pull/5937">amp-wp#5937</a>.
 * Version: 0.1
 * Author: Weston Ruter, Google
 * Author URI: https://weston.ruter.net/
 * License: GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 * Gist Plugin URI: https://gist.github.com/westonruter/35959595840823f77e4f85e584578bc3
 */

namespace Google\Try_Stylesheet_Containing_Closing_Style_Tag;

add_action(
	'wp_enqueue_scripts',
	function () {
		wp_enqueue_style( 'background-image-svg-data-url', plugin_dir_url( __FILE__ ) . '/background-image-svg-data-url.css', [], '1' );
		wp_enqueue_style( 'content-property', plugin_dir_url( __FILE__ ) . '/content-property.css', [], '1' );
	}
);

add_action(
	'wp_footer',
	function () {
		?>
		<div id="try-stylesheet-containing-closing-style-tag">
			<strong>Try Stylesheet Containing Closing Style Tag.</strong>
			This should have an AMP logo background image as well as appearing to have a closing style tag.
		</div>
		<?php
	}
);