HTML5 supports SVG via the svg element. Other languages can be embedded within SVG using the foreignObject element. HTML can be rendered in such, as can MathML. What else can be rendered with today's browsers? (Simple examples appreciated)
Example of HTML (canvas element) within the foreignObject element:
<!DOCTYPE html>
<html>
<body>
<svg id="svg1" width="200" height="100" xmlns="http://www.w3.org/2000/svg">
<foreignObject x="0" y="0" width="200" height="100">
<canvas id="myCanvas" width="200" height="100"></canvas>
</foreignObject>
</svg>
<script>
var canvas1 = document.getElementById("myCanvas");
var context1 = canvas1.getContext("2d");
context1.fillStyle = "lightblue";
context1.fillRect(20, 40, 50, 50);
</script>
</body>
</html>
Example of MathML within the foreignObject element (use FF5):
<!DOCTYPE html>
<html>
<body>
<svg id="svg1" width="300" height="300" xmlns="http://www.w3.org/2000/svg">
<foreignObject x="50" y="50" width="100" height="100">
<math xmlns="http://www.w3.org/1998/Math/MathML">
<mrow>
<mi>A</mi>
<mo>=</mo>
<mfenced open="[" close="]">
<mtable>
<mtr>
<mtd><mi>x</mi></mtd>
<mtd><mi>y</mi></mtd>
</mtr>
<mtr>
<mtd><mi>z</mi></mtd>
<mtd><mi>w</mi></mtd>
</mtr>
</mtable>
</mfenced>
</mrow>
</math>
</foreignObject>
</svg>
</body>
</html>
I prefer not to answer my own question, but it seems that the answer is: Nothing beyond HTML and MathML at this time (Aug 2011). When new info comes along, I'll remove my answer.