CSS Margin: 0 is not setting to 0

Try...

body {
   margin: 0;
   padding: 0;
}

jsFiddle.

Because of browsers using different default stylesheets, some people recommend a reset stylesheet such as Eric Meyer's Reset Reloaded.

You need to set the actual page to margin:0 and padding: 0 to the actual html, not just the body.

use this in your css stylesheet.

*, html {
    margin:0;
    padding:0;  
}

that will set the whole page to 0, for a fresh clean start with no margin or paddings.

you should have either (or both):

1. a paddding != 0 on body
2. a margin !=0 on #header

try

html, #header {
    margin: 0 !important;
    padding: 0 !important;
}

The margin is the "space" outside the box, the padding is the "space" inside the box (between the border and the content). The !important prevent overriding of property by latter rules.