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Tue, November 2, 2010 7:18pm EDT by Add first Comment

When you give an element a width of 100% in CSS, you’re basically saying “Make this element’s content area exactly equal to the explicit width of its parent — but only if its parent has an explicit width.” So, if you have a parent container that’s 400px wide, a child element given a width of 100% will also be 400px wide, and will still be subject to margins, paddings, and borders — on top of the 100% width setting. The image below attempts to illustrate this: