As jd mentions in bug 1129886, the SemanticHTML extension is a major security issue because it does not sanitize the attributes of the HTML elements it implements. And aside from that, the elements for which it adds support have long been supported natively by MediaWiki anyway.
I think this should depend on bug 716749, but I don't have access.
This has been pushed to dev: https://wiki-dev.allizom.org/Special:Version We'll go to production with it next week.
This has been pushed to stage and production. Closing and will let :GPHemsley verify and remove security flag.