Active time is based on local time, start delay and its fill mode. By spec[1], if the animation effect is in the "before phase" and fill mode is "backwards" or "both", the result should be:
  max(local time - start delay, 0).
However we return 0 unconditionally which is incorrect.

Exposing the bug requires the following and it's rare to have both:
1. negative start delay. Otherwise the effect falls into the "active phase" if local time is greater than start delay.
2. negative local time. Otherwise the effect falls into the "active phase" as well because the before-active boundary time is 0 when start delay is negative.

[1] https://drafts.csswg.org/web-animations/#calculating-the-active-time

 

Our codes do not match the spec in calculating the animation effect
phases [1] and its active time [2]. This patch updated the current
implementation by spec and fixed the related layout tests.

[1] https://drafts.csswg.org/web-animations/#before-active-boundary-time
[2] https://drafts.csswg.org/web-animations/#calculating-the-active-time