Question

Do older html files work on newer browsers?

Answer 1

will work, but it might face errors due to backward compatibility issues

Answer 2

Older html files do work on newer web browsers although they can lead to some compatibility issues in some browsers.

So the older html files may not render properly in all browsers but they will most often still work.

It's best to update your website if possible to be more compatible with newer browsers and so that the website will display properly.