The CDN integration box allows you to customize which html elements to look for.
It uses syntax from https://simplehtmldom.sourceforge.io/manual.htm as explained on the setting.
The default settings are good enough for most cases, however, if there are some static resources not being rewritten to the cdn, you can specify another rule for the replacement.
For example:
If you have a span with a background image that is not rewritten to use the cdn, something like: <span bg="https://example.com/img/new.jpg"></span>
You could target all span elements with the bg attribute with span[bg]
Or target span elements with example.com on the bg attribute with span[bg*=example.com]
Or target all span elements with span (not recommended)
The option is to specify only elements that the plugin will try to find images and replace them with the cdn.
