The \mathbin{C} and \mathrel{C} should add extra space around the character. 4/18th of an em to \mathbin and 5/18th of an em for \mathrel.
The desired behavior is discussed at
http://tex.stackexchange.com/questions/38982/what-is-the-difference-between-mathbin-vs-mathrel
The current spacing with the MathML/SVG rendering misses any extra spacing.