margin-right: 20px; /* An absolute length */
margin-right: 1em; /* A length relative to the text size */
margin-right: 5%; /* A margin relative to the nearest block container's width */
Is a <length> specifying a fixed width: it can be absolute width, e.g. in px, or a width relative to the text size, e.g. in em, or relative to the viewport size, e.g. in vh.
Is a <percentage> relative to the width of the nearest containing block.
Is a keyword indicating that the right margin receives a share of the remaining space, defined mainly by the current layout mode.. If there several auto values for margin-left or margin-right, the calculated space is evenly distributed between all the auto values. This table summarizes the different cases: