WebRender currently doesn’t support the full set of graphics primitives required to render all web pages. The focus so far has been on doing a good job of rendering the most common elements and providing a fall-back for the rest. We call this fall-back mechanism “blob images”.
The general idea is that when we encounter unsupported primitives during displaylist building we create an image object and instead of backing it with pixel data or a texture handle, we assign it a serialized list of drawing commands (the blob). For WebRender, blobs are just opaque buffers of bytes and a handler object is provided by the embedder (Gecko in our case) to turn this opaque buffer into actual pixels that can be used as regular images by the rest of the rendering pipeline.
This opaque blob representation and an external handler lets us implement these missing features using Skia without adding large and messy dependencies to WebRender itself. While a big part of this mechanism operates as a black box, WebRender remains responsible for scheduling the blob rasterization work at the appropriate time and synchronizing it with other updates. Our long term goal is to incrementally implement missing primitives directly in WebRender.