Back in the day, IE4 supported both javascript and vbscript. And from memory if you installed other eg ActivePerl (and ActivePython too I think), you could use them as well.

Since the browser will not react to these nodes, you can add some JS code that will detect them and hand off content interpretation to other mechanisms.

That's how React's JSXTransformer worked. You can also do the same for WebGL shaders:

<script type = "x-shader/x-fragment"> … </script>