The whole concept of anti-aliasing is to fool the eye via what is very similar to a Gaussian type distribution between to color surfaces meeting.
It helps give the illusion of curved surfaces being smooth by essentially blurring the edges to get rid of the jagged edges that pixel based screens create.
It is a gradient blend between the two colors.
You can't really have a blend between a color channel and an alpha channel.
Technically you should be able to have a color be "less there" meaning that if the alpha channel at that pixel has a value set up correctly (I forget if it has to go higher or lower), then that color approaches transparency (but it is up to the rendering engine of the browser, so technically different browsers very well may display it differently) - then the pixel would show more of what is behind it and therefore be more transparent.
That said - anti-aliasing only works with a fixed color - you can't do it with a transparent color.
This is exacerbated on Elite because of the alternating colors of the threads - so you can't just always have it white (as yours is not), or always have it gray - because it will show up on the other color (unless you time your posts so that they are only ever on one color).
Technically, there is likely a way you could programatically look at the color in the back and then do a real-time anti-alias on it as the color changes.
This is beyond the gif rendering engine that is built into the browsers and is more on the graphics card and OS side - and therefore is feasible for smooth text rendering and icon resizing in the OS, but not for your avatar on here.
So yeah - the short way of saying that is "choose a fixed background for your nipple".