Test mu-plugin for https://core.trac.wordpress.org/ticket/57780

test-trac-57780.php

<?php
/*
 * Test mu-plugin for https://core.trac.wordpress.org/ticket/57780
 *
 * Tests CSS URL data type support for `filter`, from this PR https://github.com/WordPress/wordpress-develop/pull/4108.
 *   This update supports https://github.com/WordPress/gutenberg/pull/48281 directly in core. This plugin:
 *   - adds a border to the image <- supported by default
 *   - adds a blur filter to the image <- this should not be passed
 *   - adds a duotone effect to the image <- confirms the patch ✅
 *
 * Installation/Removal:
 *   Copy this gist into a PHP file located in your test site's `wp-content/mu-plugins/` directory. Create directory if needed.
 *   When testing is complete, delete this file from `wp-content/mu-plugins/`.
 *
 * Before patch:
 *   Image should be displayed in full color, with only a 4px black border.
 *
 * After patch:
 *   Image should be displayed in purple/yellow duotone, with a 4px purple border. Image should not be blurred.
 */
add_action( 'after_setup_theme', 'trac_57780' );
function trac_57780() {
    if ( ! ( is_admin() || wp_is_json_request() ) ) {
    	$html = '<img src="https://placekitten.com/408/287" style="border: 4px solid #000; filter: blur(5px); filter: url(\'#wp-duotone-purple-yellow\') !important;">';
    	$allowed_html = array(
    		'img' => array(
    			'src'   => array(),
    			'style' => array(),
    		),
    	);
    	echo wp_kses( $html, $allowed_html );
    }
}