Make div fill remaining space of parent

Answer 1

Something like this might work:

http://jsfiddle.net/PCvy9/2/

The main key of what you're looking for lines in the:

 .right {
   overflow: hidden;
   background-color: #C82927;
 }

This is due to something called the "block formatting context." Great reasoning and tutorial as to why here: http://colinaarts.com/articles/the-magic-of-overflow-hidden/#making-room-for-floats

However, their heights are not completely linked; in this example, your left side block's height would still need to be manually set (as it's a floated container)