I wanted to comment on my understanding that frameset is not available in HTML5. I am currently using frameset to do an AJAX application that uses a liquid layout. I have found that using the frameset has been extremely useful for my needs.
Let me explain:
I have a user interface where there is a main frame which can adjust in size both vertically and horizontally. I have a header, footer which has fixed height but adjusts horizontally. There's also a frame to the left of the main frame that is fixed width but adjusts vertically.
So, with all of this in place I get a fluid liquid layout. With iFrame, I can adjust width / height based upon % or as fixed. However, I do not see any way to say - take up all the remaining space, like I can with the frameset.
There are three additional things that I like about the iFrame.
The browsers can handle the back button by remembering what was placed into the main frame. This is a big deal as my application is AJAX and getting back button capability like this is nice.
Since it is an AJAX application, I have lots of .css , .js and lots of global variables all of which I want to load once and then keep around and not reload for every time I need to load a new page. With the Frameset, I load all my framework into one of the frames and then load the different pages of the user interface into the main frame. The main frame therefore only loads small lightweight html and doe snot include lots of javascript.