In MathJax, it seems that nested fractions have minor problems with alignment. Specifically, they appear further to the right than they should.
It would be interesting to see if this also occurs when a fraction occurs in a sub/superscript, but I couldn't think of a way of actually checking in this scenario whether the gap was larger than it's "supposed" to be.
Consider the code
$$\frac{1}{\frac{1}{x}} \quad \frac{\frac{1}{x}}{1}$$
Here it is as rendered by the site (this is not a screenshot):
$$\frac{1}{\frac{1}{x}} \quad \frac{\frac{1}{x}}{1}$$
Here it is as I see it in Chrome 28.0.1500.95 m (zoomed in 250%):
Here it is as I see it in Internet Explorer 10.0.9200.16635 (zoomed in 250%):
Here it is as I see it when compiling it in my LaTeX editor (zoomed in 250%):
I'm using Windows 7 Home Premium 64-bit.
\Huge
command interacts with the spacing in a way such that\Huge ...
is not an accurate representation of...
zoomed up the corresponding amount. In fact, if I were really dedicated to reproducing pixel-for-pixel what I see on my screen, what I would need to do is take a screenshot while using the Windows Magnifier tool... $\endgroup$