These sources examine the architectural distinction between main-thread line layout and composited background-color animations within the Blink rendering engine. While ::first-line pseudo-elements depend on dynamic layout factors like line breaking and text shaping, composited animations are handled via native paint worklets that offload work to the compositor thread. The text explains that these two systems occupy different stages of the paint pipeline, with first-line backgrounds appearing in the foreground phase while element backgrounds are processed earlier. A narrow bridge exists between them through style invalidation logic, where the engine determines if a background change requires a full repaint or can be handled by the compositor. Ultimately, the documentation clarifies that first-line styling remains a layout-centric main-thread process that does not currently utilize the native compositor path. This separation ensures that complex text fragmentation and layout-dependent styling do not interfere with high-performance background animations.