As noted, it's a bit more complicated than just putting things above or below a symbol. Also, if you look closer at the output from MathJax, you'll notice things like the setting of x+y=z has slightly different spacing around the + than the =. For that matter, properly typeset mathematics will also the spaces around the colons differently in, e.g., f: X → { y ∊ *R* : |y| < 1 }. Math typesetting is not a simple matter. I'd also note that the existence of, e.g., ² or ₆ in Unicode is more a result of allowing the mapping of legacy encodings and that there is a preference in Unicode that in general, superscripts and subscripts not be handled through encodings but rather through the layout engine of the application which is why there is no superscripted decimal point or other such characters.
