Simple Test of Length Units (2) Margins:

Here is a regular paragraph inside a block that does not contain any extra margins, borders, or paddings. Note the positions of the apparent left and right margins.

Here is a regular paragraph inside a block that has left and rightr margins that are set in 20% -- since the parent element is the body, these margins should correspond to 20% of the width of the parent element -- namely the browser window.

Here is a second div inside the first DIV, again with 20% margins. These should be smaller margins -- and should each have a width that is 20% of the width of the parent DIV. Note, however, that the margins are actually 20% of the width of the browser window.

Here is a second div inside the first DIV, this one with -20% margins. These should be smaller margins so that the negative indent should not reach out to the edges of the window. Note, however, that they do touch the edges of the window -- since the percentage widths are calcualted incorrectly.