I don't think SVG animations should interpolate between values when animating the font-weight property. I asked on www-style whether interpolation shouldn't happen for CSS transitions here, too (so we could be consistent), but didn't get a definitive answer: http://www.w3.org/mid/8EBF3CA9-0161-4C6F-AF6F-C771FE8C4E51@gmail.com dbaron, can you confirm? This would mean that <animate attributeName="font-weight"> would always be treated as if calcMode="discrete", too.

Well, I think they should animate for transitions. Animation is relatively straightforward; it works just like animating an integer-valued property, except in multiples of 100. Why wouldn't SVG want that behavior?

That said, I guess it doesn't really matter. There's not much of a use case.

I just remember hearing that the "100", "200", etc. values for font-weight were meant to be thought of as being like keywords rather than like numbers. I agree that there's not much of a use case for interpolating here. I don't feel too strongly about it. I just want it to be the same as CSS transitions, really. The SVG spec is not at all clear about which property values types can be interpolated, although the test at the URL assumes that they are not interpolated.