Test for w3c/csswg-drafts#2717

(crossed out if not supported)
(red if not expected to be stacking context)
establishes stacking context over later inline over earlier and negative z-index positioned elements under later and positive z-index positioned elements fixed-pos containing block (should only pass for some) grouping property (based on preserve-3d) yes/no grouping property (based on mix-blend-mode in SVG) yes/no stacking context (based on mix-blend-mode in HTML) yes/no

Browser bugs