There is a TODO in PaintArtifactCompositor.cpp:
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/compositing/PaintArtifactCompositor.cpp?rcl=318723cc573ab5b141c2a40cf1874a8324f0d01e&l=394

We currently never decomposite effect nodes that have exotic blend mode.
That is because to guarantee correctness, the decomposited blend needs access to the full backdrop in the parent isolated group (effect node). That implies the decompositing decision depends on whether the layer can squash with every pending layer before it under the same parent effect, which information is not available at the time of decision.

Since we also implement mask and clip-path as blending effect, currently all mask and clip-path will be composited too. This result in text failures and some rounding differences in layout tests.