After further consideration, I have changed my mind about how the UI should work.  Having a
separate "remove" command will significantly increase the flexibility of the system.

The name for the system can be "annotate".  To create annotations for attribute tags like
alt and title, the command would be "annotate-attribute".  When annotate-attribute is called
without universal-argument, it creates transient annotations: annotations that are automatically
destroyed at the next keypress.  Transient annotations are only generated for the current
viewport, since it wouldn't make sense to generate annotations that could not be seen, if
hitting a scroll key causes their destruction.  However, when annotate-attribute is called
with universal-argument, then persistent annotations are created for the whole buffer.

Persistent annotations have to be removed explicitly with the command "unannotate", bound
on `C-x w r'.  Although for a start, we will only have attribute annotations, it is easy to
imagine that we will have other types of annotations in the future.  The unannotate command
will be able to destroy any of them, because each annotation-set will have a unique name.
For example, an annotation-set of "alt" attributes could be called "attribute:alt".  This
name could be completed on (or typed by the user) in a prompt of the unannotate command.

Any navigation action in the buffer automatically destroys all active annotations.