THE ULTIMATE CSS REFERENCE

A value of run-inmakes the element generate either a block box or an inline box, depending on the context. If the run-in box doesn’t contain a block box, and is followed by a sibling block box (except a table caption) in the normal flow that isn’t, and doesn’t contain, a run-in box, the run-in box becomes the first inline box of the sibling block box. Otherwise, the run-in box becomes a block box.