As part of work to add more spec support to the HTML API, this patch adds support for the SELECT, OPTION, and OPTGROUP elements, including the requisite support for the IN SELECT insertion mode. Developed in https://github.com/WordPress/wordpress-develop/pull/5908 Discussed in https://core.trac.wordpress.org/ticket/61576 Props dmsnell, jonsurrell. See #61576. Built from https://develop.svn.wordpress.org/trunk@58677 git-svn-id: http://core.svn.wordpress.org/trunk@58079 1a063a9b-81f0-0310-95a4-ce76da25c4cd