[development] check_markup() question

[Moshe Weitzman]

> Filtering nodes is fine, but filtering content which are not nodes is a
> little lacking, and being able make certain filters context sensitive in
> that they are only used for certain types of content would make it much
> more powerful.

@Gordon - the best way to make progress on this issue to give use cases and
ask people to solve them in an elegant way without enhancing filter system.
If no elegant solutions arise, your proposal becomes more justified. Use
cases are key.