[css3-background] Restore box-decoration-break

Cox would like to see box-decoration-break restored to this specification
rather than postponed to Level 4. In particular, we note that there appears
to be a reasonable level of implementation support already present in
common browsers. Furthermore, it is expected that TTML2 will make use of
box-decoration-break with a value of "clone", in order to map certain TTML
formatting semantics to an equivalent HTML/CSS representation [1].

[1] https://www.w3.org/wiki/TTML/changeProposal015#box-decoration-break