This presentational hint either sets the width values of all borders,
removes the bottom border or sets the height of the element, depending
on the context.
This change also implements the corresponding idl attribute (and the
`noshade` attribute, which does nothing in html5)
Testing: Adds new web platform tests
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>
Most tests were only being run for layout-2013, not for layout-2020.
This wasn't great since layout-2020 is now the default.
So this patch unifies the lists of included tests for both layouts.
For layout-2013 this implies adding css/css-content/, css/css-logical/
and css/css-masking/clip/.
For layout-2020 this implies adding several additional css tests, and
also tests like dom/, js/, html/, etc.
