Improvements to the CSS Transitions and Animations implementation

[mrobinson]

• Fix animations and transitions processing model. Also see #13865.
  • Restyle should apply animations and transitions (#26331)
  • Animation tick should trigger a restyle and properly cascade animated and transitioned values (ie getComputedStyle() should return the result of applying animations and transitions) (#26407)
  • Implement proper cascading among !important rules, animations, and transitions (#26806)
• Add support for animations and transitions to Layout 2020 (#26486)
• Better handling of animations
  • Add support for animation-fill-mode (#26519)
  • Finish the implementation of fractional iteration counts (#14858)
  • animationstart (#26659)
  • animationend (#26362)
  • animationiteration (#26594)
  • animationcancel (#26659)
  • Cascade keyframes with the same keyframe selector when computing animation steps (#26562)
• Better handling of transitions
  • Properly replace running transitions when style changes (#26540)
  • transitionrun (#26308)
  • transitionstart (#26659)
  • transitioncancel (#26244)
  • Cancel transitions for nodes that have been reparented in the DOM (#26953)
• Support transitions and animations on pseudo content (#26923)
• Add support for the Web Animations API

[mrobinson]

cc @jdm @emilio @SimonSapin

[mrobinson]

This is complete apart from the Web Animations API, which is quite a large task. I think it makes sense to close this bug in favor of #26626.

[jdm]

Fantastic effort @mrobinson!