Comment by jug

It's a language that describes how to render a document.

For example, you could describe how to render your own domain specific markup language as HTML. Which is then happily viewed by the browser as a regular webpage, that can of course also be styled with CSS as usual.

Or how to render Markdown as HTML without a Markdown generator (might require rarely implemented XSLT 2.0 with XPath 2.0). See: https://stackoverflow.com/questions/3549827/converting-simpl...

Or render HTML as some other markup language.

This is only scratching the surface, really. But it does powerful transformations from one markup language to another.

This can of course be very useful as you get the browser able to present any markup without actually supporting said markup; without scripting or server side components.