In my opinion, if one's project is not downloadable and not hostable on Itch, why have an Itch page in the first place. Itch is still a project centered store, even if many projects are free or pay what you want. So if one's project is not on Itch, something is amiss with that project page.
They kinda adress this at some places, like here https://itch.io/docs/creators/quality-guidelines#avoid-only-uploading-keys-or-li...
Of course, if one's downloadable game is on Itch and one just self hosts the web version of that game, one might get away with an alibi html5 "game". But as a user one might get suspicious and concerned if the web game one browsed on Itch is not actually on Itch, but is a download game.
What is always possible is to have a devlog/blog post about one's webgame on one's attached download version of the game.
Or put up a demo version with reduced features.
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Edit: exchanged generic pronouns with one. I know this is technically a reply, and any use of "you" might be understood as a direct form of adress to the original poster, but you cannot post a non-reply in a thread. Since we are talking about a feature for all, I though the generic context was obvious.