CSS: clear:both

assume using a container,

inside it a header,

then a left content, right side bar

then a footer

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Clearing Method

Because all the columns are floated, this layout uses a clear:both declaration in the .footer rule. This clearing technique forces the .container to understand where the columns end in order to show any borders or background colors you place on the .container. If your design requires you to remove the .footer from the .container, you'll need to use a different clearing method. The most reliable will be to add a <br class="clearfloat" /> or <div class="clearfloat"></div> after your final floated column (but before the .container closes). This will have the same clearing effect.

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Sample:
with CLEAR:BOTH

container

header

left content

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum

right sidebar

footer

without CLEAR:BOTH (Different browsers, eg.: IE, safari, chrome, firefox, will have different rendering)

container

header

left content

Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed do eiusmod tempor incididunt ut labore et dolore magna aliqua. Ut enim ad minim veniam, quis nostrud exercitation ullamco laboris nisi ut aliquip ex ea commodo consequat. Duis aute irure dolor in reprehenderit in voluptate velit esse cillum dolore eu fugiat nulla pariatur. Excepteur sint occaecat cupidatat non proident, sunt in culpa qui officia deserunt mollit anim id est laborum

right sidebar

footer