Thanks for the feedback! I did think about making fullscreen, but I wasn't sure how exactly given that I only found out about the HTML5 exporting at the last possible second :)
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Making it fullscreen should be an option on the itch editing page. If the game window can be resized in the downloadable versions, it should work perfectly in HTML5 fullscreen after a box check :)
(edit: you can also give it a manual size, which is what I ended up doing, because my game doesn't look as good fullscreen :) )