The heading does not animate back to its original font
size.
For missing styles in 0%/100% keyframes most browsers
seem to do something like "assume an implicit value
from the cascade" (with the behavior for complete
omission of the 0%/100% keyframes being the logical
extension of this). This differs from properties
missing in intermediate keyframes (see the
styles-missing-from-keyframes
example) which many browsers will tween as if the
intermediate keyframe didn't exist for the purposes
of that particular style. The recommended behavior
isn't actually specified as of writing this text
(2012-6-31). The
specification
has a placeholder which says NOTE: describe what
happens if a property is not present in all
keyframes.
.