This attribute is required by `accessibility/custom-elements/hidden-button.html` test.
<rdar://problem/104117333>
Pull request: https://github.com/WebKit/WebKit/pull/8519
Committed 258784@main (6af39536e541): <https://commits.webkit.org/258784@main> Reviewed commits have been landed. Closing PR #8519 and removing active labels.