Hmm. Could we revisit this? Especially if we change the role of filters, it'd be nice if the developer can register some filters that can bypass this restriction. The fact that two of us came up with this concept independently suggests it could have utility.

I think I agree that all Dust core filters should follow this restriction though.

(The use case is that any filter that writes HTML needs that HTML to not be escaped. It would be nice if the user's content inside was still escaped, though!)