WordPress snippet: DNS prefetch

functions.php

<?php
/**
 * Prefetch DNS for given domains
 *
 * @param array $domains List of domain names
 *
 * More info: https://css-tricks.com/prefetching-preloading-prebrowsing/
 */
function my_dns_prefetch( $domains = array() ) {
	if ( ! $domains ) {
		// Default list
		$domains = array(
		  '0.gravatar.com',
			'1.gravatar.com',
			'fonts.googleapis.com', // Google Fonts
			'ajax.googleapis.com', // Google CDN
		);
	}

	foreach ( $domains as $domain ) {
		echo '<link rel="dns-prefetch" href="//' . esc_attr( $domain ) . "\" />\n";
	}
}
add_action( 'wp_head', 'my_dns_prefetch', 1 );