Great idea, as usual killed in its infancy by lacking UA UX. We even already have an API to ask the user not to close the web page, because there's unsaved work; but it wouldn't work with "Save as...", because you can't detect that, and the browser won't do it for you, even if it knows you're editing a local file.
Same for all of the pointless cookie banners - they could've been UA prompts instead, putting the user in charge of setting a policy ("always trust example.com", "never trust example.net", "accept example.org for this session", etc). But building such prompts into a browser would've been a nuisance in 1997... So we ended up with that nuisance anyway, just enshrined by shortsighted laws, that target each and every website - rather than the three remaining browser engine vendors.
It's not killed IMHO, I actively use a fleet of such single HTML file apps that I save locally and even share with people. My initial comment has an example of a gallery, works wonderfully.
Well my issue is you forget to hit Ctrl-S/Cmd-S and your changes are gone. I've been using computers since early 90s and haven't found one program that does that; heck many apps don't even have an explicit "save" button, saving (and syncing) is transparent.
The web "browser" wasn't "intended" for this use case, hence the issue. This could be easily fixed though -- just like cookies.
You can also use localstorage/indexeddb and keep syncing it with a script element of type text/json. On-load, see if localstorage is gone (different browser, domain, etc.) and restore from the script element if that's the case.
I explicitly do not want such a thing in many of my HTML-apps, but one could add it with relative ease.
Same for all of the pointless cookie banners - they could've been UA prompts instead, putting the user in charge of setting a policy ("always trust example.com", "never trust example.net", "accept example.org for this session", etc). But building such prompts into a browser would've been a nuisance in 1997... So we ended up with that nuisance anyway, just enshrined by shortsighted laws, that target each and every website - rather than the three remaining browser engine vendors.