Look into HTML5 validation and theme updates

Let's do this. I will close this PR and open an issue to look at in Q3 2024 (July, Aug, Sept) to see if it makes sense at a later time. I agree with you on "not now".

_Originally posted by @willingc in https://github.com/pyOpenSci/pyopensci.github.io/issues/364#issuecomment-2035674304_
            
