WebGLRenderA WebGL render in progress: the display list is recorded in time slices, then drawn by the GPU in the slice that finishes it. When the recording meets something the backend does not draw, the rest of the render is the software rasterizer's, in slices too.
WebGLRendererRenders display lists with WebGL 2 in the browser: the list is recorded into triangles and commands in .NET, then drawn by the GPU in one call into a canvas the page shares, and read back into a Bitmap (or left in Canvas for a caller that composites canvases). What it cannot draw natively makes the software rasterizer draw that render instead, with a diagnostic.