The editor should be able to provide html code in a custom tag. The html should then be rendered by the browser in output view.
Since there are security aspects concerning this, the feature should be disabled by default.
And when disabled, it is just not editing that should be disabled; If a database already contains such custom tags and the features is disabled, the html should not be rendered

FYI :
In ezxml ( legacy ) this is supported using the <literal class="html"> tag.