Using <length> type values for line-height can give unexpected
inheritance behaviors. If using <length> values, the inherited
line-height on children is calculated using the font-size of the parent.
What we want is for the line-height of children to be calculated using
it's own font-size.
By instead using a unitless number, we get the behavior we want. Note
that this bug has no effects right now since no children to any of the
related elements have different font-sizes.
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