Re: [web-animations] Default fill mode

On Thu, Aug 1, 2013 at 11:15 PM, Brian Birtles <> wrote:
> Dear public-fxers,
> Fill modes are a feature available in both CSS and SVG animations that allow
> the first or last value of an animation to apply either before, after, or
> both before and after the animation itself.
> We had some trouble determining what the default fill mode should be for the
> Web Animations API, as there are good arguments for making it none, both, or
> some 'auto' value.
> On the one hand, the Web Animations API fill mode should default to 'none'
> to match the defaults of CSS and SVG Animations. This will reduce the number
> of surprises web authors encounter when refactoring code from CSS or SVG
> into script using the Web Animations API.
> On the other hand, our experience is that 'both' tends to be the fill mode
> used 80% of the time in practice when generating animations using script.
> Furthermore, when using timing groups[1], we have the following arrangement:
>   child fill mode: 'none', parent fill mode: 'both' -> no fill
>   child fill mode: 'both', parent fill mode: 'none' -> no fill outside
>                                                        the bounds of
>                                                        the parent
> In order for a child of a group to fill indefinitely in a given direction,
> both it and all its ancestors need to fill in that direction. On the other
> hand, in order for a child of a group to not fill, only that child needs to
> specify a fill mode of 'none'.
> Switching from 'none' to 'both' requires changing values throughout an
> animation tree, whereas switching from 'both' to 'none' can be done locally.
> For a more complete and graphical description, please see the attached
> image.
> We considered three options:
>  (1) choose a default fill mode of 'none' to match CSS and SVG?
>  (2) choose a default fill mode of 'both' because it makes the API easier to
> use?
>  (3) add an 'auto' fill mode that defaults to 'none' for animations
> (matching CSS and SVG) but 'both' for groups (making it easier to switch
> animations from 'none' to 'both')?
> After some discussion we think (3) is probably the best option but if anyone
> has any feedback regarding this, we would be glad to hear it.

Yes, do #3.  Groups shouldn't by default impose clamping behavior on
their contained animations, and we should match the SVG/CSS default
for the actual animations, to minimize surprise.


Received on Friday, 2 August 2013 17:59:45 UTC