html - Make div full size of window with NO margin - Stack Overflow

I hate to even ask something that seems so dumb but I just dont get this. All I want to do is have a blue div at the top of my window that stretches across the entire window. I thought width:100%;

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html - Make div full size of window with NO margin - Stack Overflow

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html - Make a div take up the full space not used by margins - Stack Overflow

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html - Cross browser method to fit a child div to its parent's width - Stack Overflow