margin (CSS property)

Description

In the normal flow of a document, margins are generally used to control the horizontal and vertical whitespace around elements. You can think of a margin as having the effect of pushing the element away from other elements on the page.

Refer to the sections on the CSS box model, collapsing margins, containing blocks, and floating and clearing to understand exactly how margins work for all elements. The section on inline formatting also explains how margins affect inline elements.

Also see The CSS Box Model for an overview of how margins are handled in relation to an element’s borders, padding, and width.

Value

The shorthand property margin allows all four sides of an element’s margins to be set using either one, two, three or four specified values. Refer to the mnemonic (TRouBLe) in Shorthand Properties as an easy way to remember the shorthand order of margins.

The property takes a CSS length (px, pt, em, and so on), the keyword auto, or a percentage of the width of the element’s containing block. Note that even for the top and bottom margins the percentage value will refer to the width of the containing block.

If the containing block’s width depends on the element to which percentage margins are applied, the resulting layout is undefined in CSS2.1. More verbosely this would be apparent for floated or absolutely positioned elements without an explicit width set, where a child element has margin expressed as a percentage. The parent element needs to know the dimensions of the child element to compute its own width (shrink-wrapping the contents), but the child box element also needs to know the parent's width to compute its margin (and hence its dimensions).

Negative values are allowed for margins (although implementation-specific limits may apply), and have the effect of pulling the element in the direction of the margin specified. This may cause the element to overlap other elements, which may, of course, be the desired effect. In cases where overlap occurs, we can determine the elements’ stacking levels by applying z-index values to them. In the case of non-positioned or floated elements, a z-index only takes effect when a position is set to relative for the elements, as a z-index can be applied only to positioned elements.

Negative margins on floats are handled differently and the details are covered in Floating and Clearing.

When you use the value auto, you’re allowing the browser to calculate the margins for you automatically. In most cases, the calculated value will default either to zero or to the distance required to reach the parent element’s edge. In the case of a block element that has a specified width, left and right margins to which a value of auto is applied will be set to be equal. This will effectively center the element in the available space.

If margins are over-constrained—that is, the total of their specified dimensions is greater than the available content space—the browser will reset one of the margins to auto to overcome the problem.

Compatibility

Internet Explorer version 6 in some instances incorrectly bases vertical margin percentage values on the width of the body element, rather than the containing block.

Internet Explorer version 5.5 (and version 6 in quirks mode) does not support the centering of a block element that has a specified width by setting its left and right margins to auto.

In Internet Explorer versions up to and including 6, the left or right margins are doubled on floated elements that touch their parents’ side edges. The margin value is doubled on the side that touches the parent. A simple fix for this problem is to set display to inline for the floated element.

Internet Explorer for Windows versions up to and including 7 differ in their handling of margin collapse from the CSS2.1 specifications. See Collapsing Margins for a detailed analysis.

If the value of the horizontal negative margins are greater or equal to the sum of the width, padding, and borders of the element, the width of the element effectively becomes zero. Some older browsers (for example, Mozilla 1.6 and earlier versions) will appear to ignore the position of the element, much as they would an absolute element. This causes issues where following elements may not correctly clear the element in question. However, most modern browsers don’t experience this issue.

Internet Explorer for Windows versions up to and including 7 don’t support the value inherit.

Internet Explorer for Windows version 8 doesn’t use the computed value of the parent’s margin when the keyword inherit is used (see James Hopkins for test case).