Or what Opera’s user.css has provided for about a decade.
View->Style->outline
View->Style->block structure
etc, etc…
Some other useful ones: View->style->table of contents – helps you verify your heading structure actually makes sense; on most sites they don’t because people are too stupid to use heading tags properly.
View->style->user mode – basically disables CSS, so if you’ve not used presentational markup like it’s still 1997, you’ll generally get an idea what search engines, screen readers and less capable browsers like Lynx have to work with. (author mode returns to normal)
View->style->Class and ID – puts the class or ID name next to the element. Often good for a laugh on websites where people have thrown endless/meaningless classes and ID’s on everything; like the garbage idiotic half-assed dumbass markup Wordpress loves to shove down your throat.
I found it to be a good in-between between notebook and dreamweaver. As it is free, I think it is worth a try. I believe dreamweaver is worth the money if you are thinking of making a lot of sites, but for occasional updating if you don’t want to spend the cash, WYSIWYG are good editors.
In fact, the only reason I have dreamweaver is because it was included with my Adobe CS stuff when I got photoshop and the like.