OpenType-SVG is a font format in which an OpenType font has all or just some of its glyphs represented as SVG (scalable vector graphics) artwork. This allows the display of multiple colors and gradients in a single glyph. Because of these features, we also refer to OpenType-SVG fonts as “color fonts”.
Support so far: Firefox 26+, Edge 38+
But good news, it's progressive enhancement friendly. The Typekit announcement page:
In browsers that lack color font support, they will fall back to regular monochrome glyphs.
So, they are usable right now. It opens up such fun doors I imagine there will be plenty of fonts coming out that take advantage of it. I'd also imagine non-supporting browsers will be wanting to get on the fun-train.
Apparently, there is a future for color bitmaps fonts, too.