The source provides a highly technical, in-depth analysis of the concept of containing blocks in CSS layout, explaining how these invisible boundaries determine the sizing and positioning of elements, particularly those with non-static positioning. The text specifically focuses on the internal implementation within Blink, the rendering engine for Chromium, detailing the differences between the older Legacy Layout System and the newer LayoutNG architecture. It thoroughly covers how various CSS properties (including position, transform, filter, and contain) establish containing blocks, how Blink's core classes handle the coordinate system lookup and layout invalidation, and how special cases like tables and fixed elements are managed for performance and correctness.
