The HTML sizes algorithm does not use full media queries. It evaluates
a restricted media-condition grammar and then parses the selected
source size value as a length.
Teach the parser to follow that split more closely: treat sizes
conditions as two-valued booleans, validate MQ5 <general-enclosed>
contents more strictly, accept calc(0) for media feature values, and
reject only source-size math results that are negative or non-finite.
The imported sizes parsing tests then progress from 140/171 to
171/171 in all four cases.
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