Investigate allowing "content" as allowed tag

Problem/Motivation

Follow up from 🐛 XSS attribute filtering is inconsistent and strips valid attributes Needs work need to look into a way to allow 'content' as an allowed attribute

From @larowlan

I think it's safe when used in an rdf context, but not in a meta element

Adding it to the list from the original ticket broke tests in StandardTest.php

Steps to reproduce

TBD

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD