I came accross this for the first time yesterday and thought that it looks like a really powerful CSS trick.
Essentially something like:
<p class="first page"> presents you with:
as a range of selectors with the latter offering a way of adding weight to the specificity.
It struck me that I have never read about this before either in any of the literature I have or here on the DW forum where I think I learned most. I'm not saying it has never been mentioned, just that I haven't noticed it.
This seemed like a good article: http://www.maxdesign.com.au/articles/multiple-classes/
Thanks for your comments Nancy and Murray.
I actually saw it after taking a look at Twitter Bootstrap and following through on some points that was raising for me. It just struck me immediately how useful it could be and it also surprised me that I had never noticed it before. I would not have noticed it in the properties panel because I tend to do most of my coding in code view, the long way. Maybe now I understand a bit more, it's time to take more interest in those DW tools too.
Isn't HTML/CSS fun!