may be related to animating css properties
visibility is a CSS property. Once bug 474049 lands feel free to reopen this if it is still not fixed. In the meantime it would be best not to raise a different bug for each of the 100+ CSS properties we don't animate.
<set> is already implemented -- it just doesn't work for CSS properties because we don't have support for tweaking them with SMIL yet. --> updating summary to indicate this.