blink::Document & blink::DocumentLifecycle

states that gate phases.

The document owns the DOM and keeps a DocumentLifecycle state machine that ensures we do things in phase order (style before layout, pre‑paint before paint, etc.). Lifecycle transitions are what UpdateLifecycle(...) advances each frame. (source.chromium.org)

Why it matters

The lifecycle enumerates phases (e.g., style recalc, pre‑paint, paint), and code uses those to assert correct ordering—critical for correctness and avoiding dirty‑state bugs during a frame. (source.chromium.org)

Hands off to: StyleEngine for style work, then on to layout and paint.