PHP8 fix and few other changes

[safraja]

There are 3 changes in this request:

1. Fix for PHP8: PHP8 does not allow use of undefined variables. Adding isset($exclude_items) to line 2404 prevents fatal error, but since $exclude_items does not exist, the IF branch is pointless and could be removed entirely. However, just in case you want to use it in the future, I decidet to solve the error with isset().
2. Addition of Webp and AVIF to the list of image formats.
3. Small fix to CSS: by specification, animation-delay always requires the unit.