How to serve WebP images with a Drupal 8/Drupal 9 site

Requirements

PHP

Your web server needs to support WebP through PHP. 
PHP 5 >= 5.4.0
GD Library

Check your PHP setup

How to check if everything is working as expected:

php -i | grep 'WebP'

If everything is working fine you should see

WebP Support => enabled

We should be ready to go on, yay 🎉

If you get another output, you might need to enable/install libgd and enable it in your PHP.ini.

Drupal

This tutorial only works with Drupal 8/9. As a start you'll need to enable/install some modules:

ImageAPI Optimize

https://www.drupal.org/project/imageapi_optimize
The module has built in support for core Image styles and a service allowing use in third party modules.

ImageAPI Optimize WebP

https://www.drupal.org/project/imageapi_optimize_webp
⚠️ Make sure to enable the "ImageAPI Optimize WebP Responsive" submodule if you're using the Responsive Image module in Drupal Core.

ImageAPI Optimize Binaries

https://www.drupal.org/project/imageapi_optimize_binaries
This module is optional but it provides support for popular binary image processors like JpegOptim, PngQuant, AdvPNG etc.

Drupal Configuration

Head over to /admin/config/media/imageapi-optimize-pipelines for the Image API configuration.

Let's create a new pipeline

1. Add a new optimization pipeline
   By clicking on + Add optimization pipeline

2. Name your pipeline
   I will call the pipeline optimize images but you can name it whatever you would like.

3. Choose your processor
   We choose WebP Deriver.

4. Set image quality
   You can set the image quality here, 80–90% should serve with good overall quality. 100% will be lossless.

5. Save
   Click on "Save configuration" and you're done.
   ⚠️ Don't forget to clear the Drupal cache after this.

You should place the WebP processor before any other compression processors to avoid multiple compression of an image. This can have an inverse affect and provide a lower quality image at a larger file size.

Enjoy WebP images with your Drupal site!