plugin.php

<?php
/**
 * Plugin Name: Reproduce #26533
 */

add_action( 'admin_bar_menu', function() {
	global $wp_admin_bar;
	$wp_admin_bar->add_menu( array(
		'id' => 'foo',
		'title' => '<span id="foo-icon" class="ab-item" style="height: 32px !important; width: 32px !important; display: block; background: red;"></span>',
		'href' => '#',
	) );
} );

add_action( 'admin_head', function() {
	?>
	<style>
	#foo-icon {
		background-image: url(data:image/svg+xml;base64,PD94bWwgdmVyc2lvbj0iMS4wIiBlbmNvZGluZz0idXRmLTgiPz4NCjwhLS0gR2VuZXJhdG9yOiBBZG9iZSBJbGx1c3RyYXRvciAxNi4wLjAsIFNWRyBFeHBvcnQgUGx1Zy1JbiAuIFNWRyBWZXJzaW9uOiA2LjAwIEJ1aWxkIDApICAtLT4NCjwhRE9DVFlQRSBzdmcgUFVCTElDICItLy9XM0MvL0RURCBTVkcgMS4xLy9FTiIgImh0dHA6Ly93d3cudzMub3JnL0dyYXBoaWNzL1NWRy8xLjEvRFREL3N2ZzExLmR0ZCI+DQo8c3ZnIHZlcnNpb249IjEuMSIgeG1sbnM9Imh0dHA6Ly93d3cudzMub3JnLzIwMDAvc3ZnIiB4bWxuczp4bGluaz0iaHR0cDovL3d3dy53My5vcmcvMTk5OS94bGluayIgeD0iMHB4IiB5PSIwcHgiIHdpZHRoPSIyMHB4Ig0KCSBoZWlnaHQ9IjIwcHgiIHZpZXdCb3g9IjAgMCAyMCAyMCIgZW5hYmxlLWJhY2tncm91bmQ9Im5ldyAwIDAgMjAgMjAiIHhtbDpzcGFjZT0icHJlc2VydmUiPg0KPGcgaWQ9IkxheWVyXzEiPg0KCQ0KCQk8aW1hZ2Ugb3ZlcmZsb3c9InZpc2libGUiIHdpZHRoPSI4OTIiIGhlaWdodD0iMjQwIiB4bGluazpocmVmPSJTY3JlZW4gU2hvdCAyMDEzLTA0LTE3IGF0IDUuMDYuMDkgUE0ucG5nIiAgdHJhbnNmb3JtPSJtYXRyaXgoMC41IDAgMCAwLjUgMjY2Ljg0ODYgLTM0OS45Njk3KSI+DQoJPC9pbWFnZT4NCjwvZz4NCjxnIGlkPSJMYXllcl8yIj4NCgk8cGF0aCBmaWxsPSIjZjFmMmYzIiBkPSJNMTAsMEM0LjQ4NiwwLDAsNC40ODYsMCwxMHM0LjQ4NiwxMCwxMCwxMHMxMC00LjQ4NiwxMC0xMFMxNS41MTQsMCwxMCwweiBNMS4zNzQsMTANCgkJYzAtMS4yNTEsMC4yNjgtMi40MzgsMC43NDctMy41MTFsNC4xMTUsMTEuMjc0QzMuMzU3LDE2LjM2NiwxLjM3NCwxMy40MTQsMS4zNzQsMTB6IE0xMCwxOC42MjdjLTAuODQ2LDAtMS42NjQtMC4xMjQtMi40MzctMC4zNTENCgkJbDIuNTg5LTcuNTIybDIuNjUyLDcuMjY2YzAuMDE3LDAuMDQzLDAuMDM4LDAuMDgzLDAuMDYyLDAuMTE4QzExLjk2OCwxOC40NTMsMTEuMDA1LDE4LjYyNywxMCwxOC42Mjd6IE0xMS4xODksNS45NTUNCgkJYzAuNTE5LTAuMDI3LDAuOTg3LTAuMDgyLDAuOTg3LTAuMDgyYzAuNDY1LTAuMDU1LDAuNDEtMC43MzgtMC4wNTUtMC43MTJjMCwwLTEuMzk4LDAuMTEtMi4zLDAuMTFjLTAuODQ4LDAtMi4yNzItMC4xMS0yLjI3Mi0wLjExDQoJCUM3LjA4NSw1LjEzNSw3LjAzLDUuODQ2LDcuNDk1LDUuODczYzAsMCwwLjQ0LDAuMDU1LDAuOTA1LDAuMDgybDEuMzQ0LDMuNjgyTDcuODU2LDE1LjNMNC43MTUsNS45NTUNCgkJYzAuNTItMC4wMjcsMC45ODctMC4wODIsMC45ODctMC4wODJjMC40NjQtMC4wNTUsMC40MDktMC43MzgtMC4wNTUtMC43MTJjMCwwLTEuMzk4LDAuMTEtMi4zLDAuMTENCgkJYy0wLjE2MiwwLTAuMzUyLTAuMDA0LTAuNTU1LTAuMDFDNC4zMzUsMi45Miw2Ljk4NywxLjM3MywxMCwxLjM3M2MyLjI0NiwwLDQuMjkyLDAuODU5LDUuODI2LDIuMjY1DQoJCWMtMC4wMzctMC4wMDItMC4wNzMtMC4wMDgtMC4xMTItMC4wMDhjLTAuODQ3LDAtMS40NDksMC43MzktMS40NDksMS41MzFjMCwwLjcxMiwwLjQxMSwxLjMxMywwLjg0NywyLjAyNQ0KCQljMC4zMjksMC41NzQsMC43MTIsMS4zMTIsMC43MTIsMi4zNzljMCwwLjczNy0wLjI4NCwxLjU5Ni0wLjY1NywyLjc4OGwtMC44NjEsMi44NzVMMTEuMTg5LDUuOTU1eiBNMTcuNTcsNS44Ng0KCQlDMTguMjQ0LDcuMDksMTguNjI3LDguNSwxOC42MjcsMTBjMCwzLjE4Mi0xLjcyNSw1Ljk2MS00LjI5LDcuNDU2bDIuNjM1LTcuNjE3YzAuNDkyLTEuMjMxLDAuNjU3LTIuMjE2LDAuNjU3LTMuMDkxDQoJCUMxNy42MjksNi40MzEsMTcuNjA4LDYuMTM2LDE3LjU3LDUuODZ6Ii8+DQo8L2c+DQo8L3N2Zz4NCg==) !important;
		background-position: 50% 50%;
		background-repeat: no-repeat;
		width: 32px;
		height: 32px;
		display: inline-block;
	}
	</style>
	<?php
} );