[Gamblor]

For the first problem its because of order and floats. You will want to have the header_navi clear: both; this should make all of the following content drop down below.

Not really sure what to say to the second problem. Margin wont change height it will just move the div down.

Third probably has to do with width and then adding a border. So lets say you have a container with 2 divs in it. The container is 500px and div1 is 300px and div2 is 200px. Fits perfect into the container. If div1 has a border all around of 1px then you would have to make div1 298px. This is because you added 2px. Thats 1px on each side. This can also become very tricky when going from firefox to IE. I would recommend grabbing a predone layout and then editing small bits. This way you know it is already cross browser compatible.