I have a div with some inline elements inside it. I want to put one of the elements on the left and the rest over on the right:
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When you float elements the parent's height is not calculated. Either you can use the clearfix class, or you can clear floats using the overflow property.
You can also add at the end of your parent div, however this is less semantic the the above solutions.
However, what you choose to use is really a personal preference.
Also you might want to try using a grid system. You can try 960 or Bootstrap.