<br> and generated-content (was: page-break properties on <br>)

It's off-topic but since we deal with <br> here, I urge
browser vendors to fully support ::before and ::after on <br>.
This allows very simple effects like

body[showBRs="true"] br::before {
   content: "\0000b6"; /* ¶ */
}

and that is important for wysisyg editors.
It also allows line numbering in a context missing element
containers for lines...

</Daniel>