Most of these do not much more than layering gradients via multiple background declarations. That gives browser developers very little room to screw up if gradients and multiple backgrounds work correctly.
It still remains a hack, of course and I guess there is little use of the technique apart from curiosity and fun.
In reality, that layering gradients via multiple backgrounds has still many quirks across current browsers: different pixel rounding errors, different blend modes often produces visually incoherent results - seams, gaps and aliasing [1].
One amusing quip: recently I have been messing with CSS3, tried the most simple use-case for `radial-gradient` I could imagine and immediately ran into Chrome bug [2] not reported at the time. I think it is somewhat illustrative.
