This box should be as wide as the browser window.

This <div> element has margins set at 20% left and 20% right.

This <p> element will be as wide as its parent, <div>.

Thus, if padding, border, and margin are set to zero, an element's width will automatically be the same as that of its parent.

This <div> element has been set to a width of 200 pixels and a height of 300 pixels.