When I decided on a local html for the app, I wanted something that did not require installing a language like python, did not require installing, and would work on Windows and Mac.
I'm not really sure what the solution is, but I'd love to be able to navigate to a site like that and know that it's not uploading my files somewhere, without having to take your word for it.
One thing that seems worse with software-on-a-webpage is the risk of malicious behaviour getting added later (this is also possible with non-browser software with automatic "updates"): you could mitigate it, if you trust or have verified one particular version of such a webpage, by saving the HTML file (put it in a Dropbox folder say) and opening the file (maybe with network disabled), whenever you need to use it. But then you lose the advantage of "don't have to install anything"; it's a trade-off.