Ping and cc @birtles

If there's already a definition for addition and accumulation of shadows, why aren't we reusing it? I don't think text-shadow should get its own one-off definition here.

This may be overly pedantic. I'm not very familiar with box-shadow and text-shadow, so it's unclear to me whether they differ enough to have different definitions for addition/accumulation. I would have thought if they were alike enough then we would have extracted a common value for them out and could define things like computed-style and animation on that?

But if a css-text-decor-3 would rather we just point at https://drafts.csswg.org/web-animations-1/#animating-shadow-lists, I'm happy to do that. I feel it is important to do something here, since Chrome and Firefox ended up with different behaviors due to this not being defined in the specs.

Committed cross-references between text-shadow and Web Animations' shadow lists. Closing out this PR. @stephenmcgruer Let me know if there's any problem (preferably by filing a new issue; I'm unlikely to notice replies here, tbh).