Of course it's extensible, you can put HTML in it, and HTML is extensible.
You can do some pretty wild stuff with html. But if you do, you get an unreadable jumble of markdown and HTML. It loses the single advantage of markdown - which is that its human readable as a text file.
It's horrible to read compared to markdown and typst.
<p>Some cool <em>sentence</em> in <b>html!</b></p>
Some cool *sentence* in **markdown!**