[web-animations-1] Allow null values for `composite`?

[birtles]

From @birtles on October 18, 2017 7:18

#148 has now been merged which allows specifying arrays of offsets. As part of this we had to continue allowing null values for compatibility reasons. On the positive side, however, you can now do this sort of thing:

elem.animate({ color: [ 'blue', 'green', 'red', 'yellow', 'pink' ],
               offset: [ null, 0.5, null, 0.8 ] }, 2000);

i.e. you can use null to represent holes that the UA fills in using its auto-spacing behavior.

Now, for composite modes, not specifying a value means "use the composite operation specified at the base level, i.e. on the keyframe effect". Being able to use null for this might be useful. For example, if you only wanted to override every second value, you could do:

elem.animate({ color: [ 'blue', 'green', 'red', 'yellow', 'pink' ],
               composite: [ null, 'add' ] }, 2000);

Maybe that's not very common, but at least it would be more consistent?

Copied from original issue: w3c/web-animations#203

[birtles]

Discussed with @graouts, @flackr, and @smcgruer. Agreed to make composite accept null for consistency.

[birtles]

Tests will be updated as part of Mozilla bug 1429671.