A run-in box behaves as follows:
Despite appearing visually as part of the following block box, a run-in element still inherits properties from its parent in the source tree.
Experimental - doesn't work everywhere
A ‘run-in’ box is useful for run-in headers, as in this example:
h3 { display: run-in }
<h3>A run-in heading.</h3>
<p>And a paragraph of text that must not be in another line</p>