Adds async and preload behavior to a given stylesheet in a WordPress theme. Opt-in only.

async-preload-stylesheets.php

<?php
add_action( 'style_loader_tag', function ( string $tag, string $handle, string $href, string $media ) : string {
	$allowlist = [ 'lwtd-style' ];

	if ( !in_array( $handle, $allowlist, true ) ) {
		return $tag;
	}

	$new_tag     = new WP_HTML_Tag_Processor( $tag );
	$fallback    = '<noscript>' . $tag . '</noscript>';
	$preload_tag = '<link rel="preload" href="' . $href . '" as="style">';

	$new_tag->next_tag('link');
	$new_tag->set_attribute('media', 'print');
	$new_tag->set_attribute('onload', "this.media= '$media'");

	return $preload_tag . $new_tag->get_updated_html() . $fallback;
}, 99, 4 );