How do modern frameworks display content? Must be interacting with HTML and manipulating the DOM in some way.

It's not whether they do, it's how they do.

For instance, in 2003 there was no such thing as a mobile phone; passing code review today is probably going to entail either being responsive, or directing to a mobile version when accessed from a mobile device.

Okay, I will clarify my hyperbole in the face of the pedantic - mobile phones did not provide ubiquitous, standardized approaches to browsing the web, such that UX guidelines for the web did not include them; media queries and such did not exist except as drafts, etc etc.