You could also try:
Code:
<h3 title="Chapter 13. Blah, Blah, Blah">Chapter 13.</h3>
<p class="subheading">Blah, Blah, Blah</p>
Then it doesn't matter what you put between the heading tags as what is recorded as the title will be used for the TOC.
So if you have a multi-line heading that uses different formatting for the chapter number and the rest (e.g. different font sizes, etc) you can have a bit more control on the layout. At least that's what I've found.