1. Give div width "auto" and a fixed height.
2. Add content to div such that it has width 10px
3. Set css width transition on div.
4. Set div width to 400p.
The draft doesn't say anything about transitions from/to auto so I'd expect no transition to occur and for the div to immediately grow in size.
The div immediately shrinks to a width of 0 and then grows in size to 400px. This is pretty ugly.
*** This bug has been marked as a duplicate of bug 16020 ***