The Body HTML Element and its Margins

Usually, the background of an HTML element only fills the content area of the element, not the margins. If you set a background color on a p element and give it a margin-left of 10px, that margin remains the color of its parent element, not the backgound color of the paragraph.

This is not the case for the body element.

Since the body element represents the content of the entire page, its margin has a little different meaning. If you set a background color on the body element and give it a margin, the color of the margin is that color.

A way to think of it is that the margin of the body element is not about positioning the body element relative to other elements (since it is already the top level element) but about setting a document-level margin for its child elements.