Form element appears to be distorting parent div margin/padding

I created a test page to demonstrate what is happening:

Below the "submit order" and "edit account information" buttons is a white space which I don't want to be there. The space is in-between the elements div#content and div#bottomswirls. It appears to be related to the form element on this page. If I take away the form element, the space goes away.

However, I don't understand why it is there, or what I can do to get rid of it. My margin and padding are set to zero already.

Any advice?
Thank you!
Brad BansnerWeb DeveloperAsked:
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Tom BeckCommented:
main.css line 10, paragraph margin under the buttons, 1em on the bottom.

p, ul, ol {
    margin: 0 0 1em;
Brad BansnerWeb DeveloperAuthor Commented:
OK. I sort of understand. But why should the margin at the bottom of a paragraph tag cause the background color (which is on the parent DIV element) to have a margin?
Tom BeckCommented:
You have a min-height set for the parent div. The content exceeds that minimum so the last paragraph is being forced to the bottom. In the css box model, margin is the outermost box and it is applied outside of the container's borders as well as outside the borders of its parent container. If you want the space between paragraphs to be applied inside the parent container, use padding.

The css box model:

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Brad BansnerWeb DeveloperAuthor Commented:
Changing margin to padding does seem to fix it. Thanks!
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