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https://drafts.csswg.org/cssom/#serializing-css-values

"To serialize a CSS component value depends on the component, as follows:
keyword
The keyword converted to ASCII lowercase."

Bug: 901666
Change-Id: Iff579a363de13441c75bbebfc951095b071f6060
Reviewed-on: https://chromium-review.googlesource.com/c/1317300
Commit-Queue: Emil A Eklund <eae@chromium.org>
Reviewed-by: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Emil A Eklund <eae@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605383}