When it encounters:
<script type="byte/code" language="bytecode" src="http://myserver/somecode.byt"></script>
it will pass it to the bytecode interpreter to run.
When it encounters:
<script type="text/javascript" language="javascript" src="http://myserver/media/js/jquery.js"></script>
it will run it as it currently does through the JS interpreter.

The browser continues to do the work _for JS_, but allows you to write code in other languages if you so desire.