I thought kerning specifically referred to small adjustments in spacing between pairs of letters. This just seems to be a failure in honoring the spacing for the different TeX math mode symbol classes. The rel and op classes each have a certain amount of spacing they are supposed to have on either side (unless they aren't used as binary operators), and somehow this is broken in their implementation -- the spacing doesn't come from the font configuration.

For example, a\mathbin{foo}b is supposed to render as "a foo b", but on GitHub it comes out as "afoob".

When I looked at the MathJax configuration GitHub uses, nothing pops out as being odd. It would be funny if a minifier totally messed up the part of the MathJax source code specifically for this.