percentage padding/margin: top/bottom percentages resolve against the
containing-block width (the responsive aspect-ratio-box trick, e.g.
padding-top:56.25%), resolved to px in the top-down layout pass so card
media boxes reserve space instead of collapsing.
box-shadow: parse and paint an outset shadow (rounded, offset, spread,
soft-edge approximation) so cards/buttons/menus have depth.
external-sprite svg: resolve <use href="url#id"> to an external sprite
file, inject the referenced symbol so inline-svg icon systems rasterize.
background-clip:text: paint transparent-with-gradient text (the common
gradient hero/button label) instead of leaving it invisible.
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