[css-transitions] Fix reasonable bikeshed fatal error:  Convert <td><em>...:</em></td> in propdef tables to <th>...:</th>.
[css-flexbox] Clarify what is meant by white space

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extra : rebase_source : c4d28cef18c13010872ec167a409c38ef2d86239
[css-flexbox] Update DoC

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extra : rebase_source : 663692526174f7606fc28988bd26ddb6562e36e9