NOTE for Silverstripe 5.3

[elliot-sawyer]

If you're using Focuspoint in your templates and want to convert to webp, it needs to be the last call in your template. For example:

$FeaturedImage.FocusFill(880, 480).Convert('webp') will work, while:
$FeaturedImage.Convert('webp').FocusFill(880, 480) will not work

[elkebe]

Note that using
$FeaturedImage.FocusFill(880, 480).Convert('webp')
will result in a larger file size than using eg.
$FeaturedImage.Convert('webp').Fill(880, 480)