Take shadow DOM into account

[frivoal]

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Migrated from WICG/spatial-navigation#21
Originally created by @frivoal on Fri, 16 Feb 2018 06:28:28 GMT

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The spec does not currently say anything about shadow DOM. It most certainly should say something.

[jihyerish]

The issue migration script created some duplicates. #3381 is the same as this one.
So I'll close this issue.