If I understand correctly, the <percentage> in animation-range-start and animation-range-end refers to a specified point on the timeline. Currently, when declaring animation-range with a single <timeline-range-name>:

If a <timeline-range-name> alone is specified, animation-range-start is set to that name plus 0% and animation-range-end is set to that name plus 100%.

So I wonder why animation-range-start and animation-range-end do not allow <percentage> to be optional:

  Name:	animation-range-start 
- Value:	[ normal | <timeline-range-name> <percentage> ]#
+ Value:	[ normal | <timeline-range-name> <percentage>? ]#

It would default to 0% in animation-range-start and 100% in animation-range-end.

Also, I guess this <percentage> should be restricted to [0,100], so <percentage [0,100]>, as well as in <keyframe-selector>.

I also wonder if:

• <percentage> should always be lower in animation-range-start than in animation-range-end
• normal should not be allowed to repeat
• it would be more intuitive to expand animation-range: timeline-1 5% to animation-range-start: timeline-1 5%; animation-range-end: timeline-1 100% (instead of animation-range-start: timeline-1 5%; animation-range-end: normal
• <timeline-range-name> is inappropriate in <keyframe-selector> because I think animation-range defines an interval for all keyframes and an author is not supposed to declare eg. timeline-a 0% {} timeline-b 100% {} with animation-range: timeline-x

But these questions might deserve their own issue and I realize that these properties are currently under discussion so they may be a bit premature.