The W3C validator is rather unhappy with python.org, as one can see here: https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.python.org%2F

Now, I don't believe in "validation for validation's sake", but at least some of these look potentially serious to me -- in particular, the invalid "role" attributes. As these are used for screen readers, I wonder if this breaks screen some screen readers, and thus might impair accessibility?

Fixing those shouldn't be hard ... well, at least to fix the HTML -- I have no idea how this site is implemented, so perhaps actually making the change is much harder.