You can look through this list for games that have the little "playable in browser" badge, but that doesn't appear to be a search filter option. You can also remove the open source tag from the filter to get more results.
Here are direct links to some examples
https://mahda.itch.io/scream
https://alzter-s.itch.io/supertux-classic
Edit: see the comment from sxx, they found the right filter. There is a "platform" field that can be set to web, separate from the tags.