Resource Hints in 4.6

Resource Hints is a rather new W3C specification that “defines the dns-prefetch, preconnect, prefetch, and prerender relationships of the HTMLHTML HyperText Markup Language. The semantic scripting language primarily used for outputting content in web browsers. Link Element (<link>)”. These can be used to assist the browser in the decision process of which origins it should connect to, and which resources it should fetch and preprocess to improve page performance.

Introduced with [37920], WordPress now has a simple APIAPI An API or Application Programming Interface is a software intermediary that allows programs to interact with each other and share data in limited, clearly defined ways. to register and use resource hints. The relevant ticketticket Created for both bug reports and feature development on the bug tracker. is #34292

By default, wp_resource_hints() prints hints for (the The community site where WordPress code is created and shared by the users. This is where you can download the source code for WordPress core, plugins and themes as well as the central location for community conversations and organization. CDN) and for all scripts and styles which are enqueued from external hosts.

Developers can use the wp_resource_hints filterFilter Filters are one of the two types of Hooks They provide a way for functions to modify data of other functions. They are the counterpart to Actions. Unlike Actions, filters are meant to work in an isolated manner, and should never have side effects such as affecting global variables and output. to add custom domains and URLs for dns-prefetch, preconnectprefetch or prerender. One needs to be careful to not add too many resource hints as they could quite easily negatively impact performance, especially on mobile, but the filter works like this:

function makewp_example_resource_hints( $hints, $relation_type ) {
	if ( 'dns-prefetch' === $relation_type ) {
		$hints[] = '//';
	} else if ( 'prerender' === $relation_type ) {
		$hints[] = '';

	return $hints;

add_filter( 'wp_resource_hints', 'makewp_example_resource_hints', 10, 2 );

#4-6, #dev-notes, #script-loader