[neilv]

You could rig up a proxy that turns HTTP requests into HTTPS (when needed) and also rewrites `https:` scheme URLs in HTML to `http:`.

If your vintage browsers support some old JS, you could also have the proxy to do a a very simple kludge to patch JS going through it to fix some URLS, but not all (because The Halting Problem).

Bonus of a transforming proxy: if there are bad known vulnerabilities in some vintage browsers that can't be fixed, the proxy might be able to filter at least some exploits. (But you might not tackle cross-origin vulnerabilities in any case, so keep that in mind.)