You’ll have to subtract the height of the navbar from the 100%. There are several solutions, many of which will include JavaScript but you won’t need that.
1: box-sizing
Give the navbar an absolute position. Then you have the issue of the content of the other elements disappearing below it.
Then comes the next trick: add a top-padding the size of the navbar.
And the last trick: add box-sizing: border-box; to the .sidebar and .body. For better browser support you also need -moz- and -webkit- prefixes like so: -moz-box-sizing:
Example: Fiddle to box-sizing
2: Actually subtract.
use .body, .sidebar{ height: calc(100% - 40px);
Browser support is very minimal for this from what I know less than for box-sizing, so I would recommend the first solution.
Calc explained on css-tricks
3: Flexbox(added anno 2015)
You should now probably go with using calc when you know the height but an awesome replacement for using tables is flexbox. This is really my saviour for complex designs in responsive websites. By using one of the best features – flex-shrink: 0 – on the header you can force other elements into adjusting themselves to fill the rest of the container.
An old answer is probably not the best place to write down an extensive guide on flexbox so, again, a great link to css-tricks