Extract text emphasis style
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Extracted the text-emphasis-style property out of the inherited_text.mako.rs.
This is a part of #19015
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Much like we optimize to_ascii_lowercase.
This also fixes a bug in Servo where attr() rules with an unknown namespace
prefix are parsed, which is wrong.
Change debug assertions to specific ones
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style: Move content property out of mako.
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r? emilio
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