According to https://html.spec.whatwg.org/#case-sensitivity-of-selectors,
the CSS selector attribute name must be converted to lowercase in HTML elements,
and then compared case-sensitive to the attribute name in the element.
We implement this not by doing the explicit conversion, but by a manual
loop using a function that first converts the rhs characters to
lowercase and keeps the lhs characters the same, achieving the same
effect.
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