Comment by InfiniteVortex

Author here (I'm not OP): It converts filter lists to Safari's content blocking API using SafariConverterLib (by AdGuard) and stores advanced blocking (scriptlets, extended css, css & js) in a json file. The WebExtension then reads the json file through native messaging and injects the scriptlets, extended css, css & js. This means that things like YouTube ads can be blocked too, whereas using just the content blocking api won't allow YouTube ads to be blocked