It turns out that aside from HTTPS, most older browsers can do quite well with HTML websites! If you do it right, all old browsers will be able to render it because it'll work in WorldWideWeb.app.
However, if you want universal html (no changing the html for individual browsers) and some half-decent web-app functionality and no-js compatibility and accessibility and for it to look pretty, be prepared to spend many hours tweaking it.
Here's a video of some testing I did recently. Here, I discover that IE doesn't play nice with being forwarded to a URL which has an anchor at the end.
