If I buy GameGuru Loader for AppGameKit Classic, will I be able to export this game as HTML?

based on preben's ggloader>html demo HERE, i'd say yes.

one thing i'd be aware of is that HTML exports don't deliver precisely the same results compared to other builds. ie, see here for some differences. now, i don't know how preben has handled "shadows" there (i think the source is somewhere) and i have zero experience with GGLoader but GGLoader maybe improve the HTML experience in some ways?

there may be other examples somewhere but that's the one that i recall and i don't know the current state of things (ie, how recent versions of AppGameKit might get along with GGLoader), so if anyone has a recent export/test, let us know?

It used to work so I assume it still do, not tried for some years, anyway if you purchase it and it dont work in classic you can get a refund , so give it a try

And yes it do support shadows in html5 it have some special webgl shaders for this, just enable it "global loadwebglshaders = 1".