EWWW IO and WebP Images
Unlike the other conversion options in the EWWW IO plugin for WordPress, WebP conversion is possible for every PNG or JPG image on your entire WordPress site. It is not limited to the Media Library. So theoretically, you could put the root path to your WordPress site in folders to optimize, run a Bulk Optimize, and EWWW would create WebP copies of all images where WebP is the smaller of the two formats.
JPG/PNG to WebP
Converting JPGs to WebP involves a slight quality loss because we use the lossy WebP encoding for JPG images. On the other hand, converting a PNG to WebP is completely lossless, so you don’t have to worry about losing quality there. The WebP conversion does NOT replace your images, it only makes a copy of the original with the exact same name, but in the WebP format and with a webp extension added onto the end.
It is important to note that only the Chrome and Opera browsers, and Adroid devices currently support WebP. so determining how to serve those images is somewhat up to you, but there are a couple options available in the plugin. Also, by default, the plugin will only create the WebP images if they are smaller than the original format (JPG or PNG), so any rewriting solution has to check to see if a webp image exists, and that just isn't possible if the only copy of an image is on a remote server.
Rewriting with Apache & Litespeed
The first solution is for folks using Apache (or Litespeed) with mod_headers and mod_rewrite. The EWWW IO plugin can attempt to modify the rewrite rules in your .htaccess file to send WebP images to supported browsers (if such a file exists). When you enable WebP conversion, you’ll see a new section at the bottom of the settings page. It includes the rewrite rules for Apache, a button to insert those rules for you, and a red PNG image showing you that either the server is not sending WebP images, or your browser does not support them. When you insert the rules, you will get a success message if it worked, and it will attempt to reload the test image as well. It may sometimes be necessary to reload the page to get the right test image to load. When you're using a native Apache or Nginx rewrite solution like this, you won't see the image urls in your page change. The rewriting happens when the visitor's browser actually requests the image.
This video demonstrates the two methods of serving WebP images, and how to verify that they are working:
If you're running a multisite WordPress install, you may have to put the rewrite rules above your Wordpress rules for them to take effect. And if the rewrite rules aren't working, you can try the Alternative WebP Rewriting instead.
Alternative WebP Rewriting
Force WebP & WebP URLs
So, what are these other two options for? Normally, none of the solutions we’ve talked about will work if you do not have local copies of your images. To get around that requirement, you first need to use the Force WebP option to make EWWW IO create WebP versions of every image you optimize. Then, you need to create a whitelist of domains or urls that EWWW is allowed to rewrite, and enter these URLs in the WebP URLs box. They can be partial urls, but should contain at least the domain name where the images are being served from.
Once those two options are setup, you can enable Alt WebP Rewriting, and then the plugin will look for any images that match the url patterns you’ve listed, and it will rewrite those images for WebP. It still provides a fall-back for browsers like Firefox and Internet Explorer, but you need to make sure that every image matching these urls already has a webp version, or you’ll end up with broken images in chrome and opera.
AMP (Accelerated Mobile Pages)
AMP has this fabulous 'fallback' attribute that allows you to wrap one image element inside another. Then if the first image cannot be displayed, the second one loads instead. The AMP Project Docs even show an example of this, but it doesn't really work properly. The problem is that unsupported browsers will still load the webp image, and only then, after you've wasted precious mobile data, does it load the original (fallback) image. I wish it were not so, but until all AMP browsers support WebP, we'll just have to leave them untouched.