There should be some way to mark files as accessible, for example, place them into a folder with a specific name (like "html-accessible-files") on the same level as HTML file.

Internet Explorer in an ancient age used to have .HTA ("HTML Application") files for a double-click a self-contained HTML file that can act as a small local application. It did a lot of what PWAs are still trying to do, somewhat more effectively and simpler. (.HTA was mostly just a ZIP file! Simple to build.) It also had a lot of security holes that gave it a terrible reputation and a lot of reasons it got killed. (It was very early days for "AJAX" and modern browser security tools like CORS and whatnot, after all.)

Such things seem to be cycles.

Today a lot of browsers support .MHT which is a similar format, but also worse in many other ways. (The M stands for MIME and wrapping a website like an email seems somehow sillier and weirder to me than wrapping it in a ZIP file, though I get that MIME wrappers are ancient internet tech with an ancient track record.)

Then we see all the millions of apps in PWAs and Electron downloads.

At some point it feels like we should have better solutions and cut some of the gordian knot cycling between "local apps are too much of a security risk" and "local apps should be complicated collections of Service Workers to get offline support" and "local apps should just embed a full browser and all its security implications/risks rather than allowing browsers to directly open local apps" and back and forth. .HTA and .MHT both showcase possible directions back to "simpler" than PWAs/Electron, they just have such fascinating and weird histories.