Add test
Add test for 5121: Blending simple elements
Add test for 5122: Blending video with sibling. Remove older test: blending_video.
Add test for 5123: Blending with overlapping sibling. Rework older test: mix-blend-mode-blending-with-sibling-ref.html.
Add test for 5124: Blending element is interposed. Rework older test: mix-blend-mode-blending-two-levels-of-ascendants.html.
Updates
Remove old code
Add text/css for styles
Add test
Address review comments
Remove smart quotes
Update
Address review comments
Update video test ref title
Add test 5131
Add 5132
Add 5133
Fix line endings
First step for 5141
Rework 5141