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Couldn't reproduce the reported "hero-yes doesn't show" in a live WebKitGTK check (svmin/svh both resolved correctly there), but an engine that doesn't support these newer viewport units would treat the whole declaration as invalid and drop it - for .hero-yes's height that means falling through to auto, which collapses the section to its in-flow content height and clips the canvas via overflow:hidden. Declaring a plain px/vh value first means an unsupported engine keeps that instead of falling through further.