I always found it interesting that style tags were designed as <style type="text/css">, implying that new style standards could be introduced down the road.
(for a while I thought <script type="text/dart"> might get real… oh sweet naivety)
I have a toy which allows <script type="text/smalltalk">. The scripts get transpiled on-the-fly to Javascript.
(Although the process you have to go through to hook a new script type is painful. There are, like, four different cases you need to consider, and scripts have to be processed in the right order but handled synchronously, and...)
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.
(for a while I thought <script type="text/dart"> might get real… oh sweet naivety)