SVG? Vectorized, far better browser support than advanced CSS, advanced WYSIWYG editor support.

SVG is another obvious contender, but it needs a separate file (or a cryptic data url), which is not always a disadvantage, but sometimes may be.

It seems like CSS has many of the same possibilities as SVG, although expressed a bit more obscurely. But I can imagine that an SVG-to-CSS converter would be possible.