[web-animations-1] Clarify consequences of mis-sorted keyframes

[fantasai]

There's a (non-normative) section of the spec that says

offset
...
Specifying an offset outside the range [0.0, 1.0] will cause a TypeError to be thrown.
Keyframes that specify an offset must be provided in increasing order of offset. Adjacent and equal offsets, however, are permitted.

If that MUST is violated, what happens? Would be good to say something here.

[birtles]

It throws a TypeError as per step 3 of process a keyframes argument.