In https://gerrit.wikimedia.org/r/c/mediawiki/services/parsoid/+/952947, @cscott wrote,

So the way this seems to work in core is that you call MagicWordFactory->newArray(...) with an allow-list of canonical keys, and then MWF matches only against aliases of those keys. Further, that list is extensible by MediaHandler->getParamMap(), so we should probably try to duplicate that functionality.

In T208578#9288476, it's noted that the Tiff Handler's lossy parameter isn't recognized by Parsoid.