I wish to second the notion that using floor() to round to integer values creates a more intuitive animation behaviour from the point of view of a web developer.

I have also identified that at present, Webkit is contravening the W3C (draft) spec by using round() for CSS Animation of integer properties - presumably they are of the opinion that both SVG/SMIL and Animation/Transition should use round() to animate integer properties - I personally disagree (and instead agree with Brian).

http://code.google.com/p/chromium/issues/detail?id=128534

EDIT: I misread Brian's original comment.

So in fact I disagree with Brian and agree those who originally wrote the W3C CSS Animation spec. It is my opinion that using floor() to round integer properties creates a more intuitive animation from the perspective of a web developer.

I propose that the SVG/SMIL spec should be adjusted to specify floor() rounding for integer animation.

I have identified one situation in which floor() rounding allows for a much more elegant CSS animation solution to a simple problem.

http://dabblet.com/gist/2760288

I wish to propose that if/when the specification is changed to specify round() for the rounding of integer property animation/transition that it should simultaneously be changed to allow non-integer transition/keyframe values for integer properties and that these should be rounded only after interpolation has taken place.

In this way, a web designer would be able to create a floor() like behaviour simply by subtracting 0.5 from their keyframe/input values. At present the latest Webkit Nightly simply ignores non-integer input values for integer property animation.

(In reply to comment #3)
> I have identified one situation in which floor() rounding allows for a much
> more elegant CSS animation solution to a simple problem.
> 
> http://dabblet.com/gist/2760288

You can achieve the desired effect even more elegantly using round by simply omitting the middle keyframe value for z-index:

@keyframes orbit {
    0% { margin-left: -150px; z-index: 10; }
    50% { margin-left: 100px; }
    100% { margin-left: -150px; z-index: 9; }
}

I'm sure it is possible to find some cases where floor more elegant. Likewise, there will also be some where round is more elegant.

The main issue is that if we make CSS use floor it will be very hard to make animations interchangeable between CSS and SVG (which is a Rec).

If you discover some really compelling reason for using floor then I'd encourage posting it to public-fx.

I understand the obvious advantage of having animation&transition in line with SVG/SMIL. I was not proposing that floor() should be kept.

I was instead proposing that non-integer inputs should also be allowed.

(In reply to comment #5)
> I understand the obvious advantage of having animation&transition in line with
> SVG/SMIL. I was not proposing that floor() should be kept.

Ok, sorry I misunderstood.

> I was instead proposing that non-integer inputs should also be allowed.

I think you achieve the desired effect by simply omitting property values from keyframes. This is something that's not clearly specified at the present time. There is a note in CSS Animations saying, "describe what happens if a property is not present in all keyframes" but I think when it is spelled out, it will give the effect desired in this case.

Fixed in https://dvcs.w3.org/hg/csswg/rev/54cd45623730