The provided sources detail the evolution and implementation of the Web Animations Level 2 (WA2) specification within the Chromium Blink rendering engine. Web Animations Level 2 is framed as a supplemental update rather than a full replacement, introducing a shift from simple time-based animations to a hierarchical tree model and progress-based timelines. The documentation distinguishes between stable features already integrated for scroll-driven animations, such as overallProgress and range-based offsets, and unstable components like grouping, sequencing, and exploratory custom effects. For Blink engineers, this transition necessitates a major architectural expansion to handle complex timing inheritances, non-monotonic timelines, and new compositor-thread synchronization challenges. The sources ultimately emphasize a staged implementation strategy that prioritizes finalized timing standards while deferring more speculative and high-risk elements of the draft.