Add last missing document structure element from SVG: <defs>, a hidden container, not directly participating in hit-testing / painting.
Pull request: https://github.com/WebKit/WebKit/pull/3027
<rdar://problem/98457319>
Committed 253510@main (a52e110542e4): <https://commits.webkit.org/253510@main> Reviewed commits have been landed. Closing PR #3027 and removing active labels.