Send preload headers for WordPress CSS files

functions.php

<?php
/**
 * Preload styles
 *
 * @return void
 */
function mytheme_preload_styles(): void {
    global $wp_styles;

    if ($wp_styles) {
        $site_url = get_site_url();

        $urls = [];

        foreach ($wp_styles->queue as $style) {
            $data = $wp_styles->registered[$style];

            if ($data->src && ! isset($data->extra["conditional"]) && ! (isset($data->args) && $data->args !== "all")) {
                $src = $data->src;

                if (strpos($src, $site_url) !== false) {
                    $src = str_replace($site_url, "", $src);
                }

                if ($data->ver) {
                    $src .= "?ver={$data->ver}";
                }

                $urls[] = "<{$src}>; rel=preload; as=style";
            }
        }

        if ($urls) {
            header("link: " . implode(", ", $urls));
        }
    }
}
add_action("send_headers", "mytheme_preload_styles", 1, 0);