On the ‘missing scheme’ thread, @jgm said:
The heuristic I would propose is that if the contents of the angle-brackets contains a colon and contains no whitespace, it should be treated as a URI and a hyperlink generated, rather than treating it as a tag.
A namespace-qualified tag like <m:math> alone doesn’t have much meaning standing alone, and to declare the prefix requires spaces inside the angle brackets. This is reflected in MathML documentation, for example the MathJax documentation says:
Also note that, unless you are using XHTML rather than HTML, you should not include a namespace prefix for your
<math>tags; for example, you should not use<m:math>except in a file where you have tied the m namespace to the MathML DTD by adding the xmlns:m=“MathML Namespace” attribute to your file’s<html>tag.
So for <m:math> to mean something alone, it needs a declaration. The namespace-prefix declaration mechanism I suggested in the Compact URIs thread could be used to give a prefix like m an extra semantic kick that caused it to be passed through unaltered. E.g. a prefix declaration like this might be used in a CommonMark document:
<? prefix m: http://www.w3.org/1998/Math/MathML !verbatim ?>