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.
In some cases the spacing should depend on the context. The ordinary plus sign is a binary operator, and when one types "5+3", the spacing to the right of "+" is conspicuously larger than what it is when one types "+3", using "+" as a unary operator. Similarly in a\ln b and a\ln(b), the spacing to the right is different: much smaller in the latter.
This is a texvcjs bug.
A \mathrel{C} B turns into A{\mathrel {C}}B (same for \mathbin) and that messes up TeX layout.
The output is the expected result for the texvc output.