Making things work on crappy browsers is definitely important. But that doesn't always mean "simple HTML", and it definitely doesn't mean avoiding react or jQuery or whatever HN framework hipsters are complaining about this week - if anything, those popular, "bloated" frameworks are more likely to have been tested on these obscure browsers and have whatever workarounds you didn't think of. It might not even mean following "web standards"; frankly I've found writing HTML3 with no CSS, table layouts, and maybe even the occasional <EMBED> works a lot better on these old browsers than flexbox, semantic tags, or whatever the overengineers are trying to push these days.

The bottom line is right; rather than posturing, test your site - actually test it - on one of these browsers.