> You can have a document without CSS but you can’t style it.

What? CSS didn't come around until several years after HTML did. And you can certainly style an HTML document without CSS.

> On the other hand, you can replace XSLT with server side rendering, or JavaScript.

You can also execute JavaScript on the server to make browsers more secure, but I don't see browser makers clamoring to remove JavaScript support.

> It does not serve a truly unique function.

It does, though. It lets someone do some basic programming of some web pages without having to become a developer

Inline styles came and went and were replaced by CSS. (style attribute is still just CSS). font, color, and others are no longer in HTML5 spec.

> You can also execute JavaScript on the server to make browsers more secure, but I don't see browser makers clamoring to remove JavaScript support.

JS is not there just for client side static DOM rendering. Something like Google Maps or an IRC chat would be a much poorer experience without it.

> font, color, and others are no longer in HTML5 spec.

Sometimes browsers are asked to render HTML documents that were written decades ago to conform to older specs and are still on the internet. That still works

> JS is not there just for client side static DOM rendering. Something like Google Maps or an IRC chat would be a much poorer experience without it.

Of course they would. That's most of the point. You can do a lot more damage with JavaScript than you currently can with XSLT, but XSLT has to go because of 'security concerns'