Feedback from Evan Wallace:

 It’s too bad that CSS shaders can’t really be used for compositing two DOM elements together. This would allow for custom blend modes and custom transitions. While some transitions can still be done using the default alpha compositing in the browser, this limits the things that can be done with CSS shaders. This can sort of be overcome by nesting the two elements that you want to composite together under one element and doing two texture lookups in the shader but is harder than it needs to be. I’m also not sure if this hack works for compositing two or more screen-sized elements because you would need a texture at least twice the size of the screen.