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* GPU commands execution
All of these phases are performed on different threads. The first two happen whenever the layout of the page change changes or the user scrolls past the part of the page covered by the current scene. They don't necessarily happen at a high frequency. Frame building and GPU command execution, on the other hand, happen at a high frequency (the monitor's refresh rate during scrolling and animations), which means that they must fit in the frame budget (typically 16ms). In order to avoid going over the frame budget, frame building and GPU command execution can overlap. scene building and frame building can also overlap and it is possible to continue generating frames while a new scene is being built asynchronously.
WebRender has a fallback mechanism called "Blob images" for content that it does not handle (for example some SVG drawing primitives). It consists in recording and serializing a list of drawing commands (the "blob") on the content process and sending it to WebRender along with the regular display list. Blobs are then turned into images on the CPU during the scene building phase. The most of WebRender treats blob images as regular images.
