<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>CSS Test: Margin with a value of 'auto'</title> <link rel="author" title="Microsoft" href="http://www.microsoft.com/" /> <link rel="reviewer" title="GĂ©rard Talbot" href="http://www.gtalbot.org/BrowserBugsSection/css21testsuite/" /> <!-- 2012-08-13 --> <link rel="help" href="http://www.w3.org/TR/CSS21/box.html#propdef-margin" /> <link rel="help" href="http://www.w3.org/TR/CSS21/box.html#margin-properties" /> <link rel="match" href="margin-005-ref.xht" /> <meta name="assert" content="The 'margin' property with a value of 'auto' correctly sizes the margin. In normal flow, 'margin-top: auto' and 'margin-bottom: auto' for block-level, non-replaced elements are resolved as 0. In normal flow, 'margin-left: auto' and 'margin-right: auto' for block-level, non-replaced elements are resolved according to the constraint equation for the width of containing block: 'margin-left' + 'border-left-width' + 'padding-left' + 'width' + 'padding-right' + 'border-right-width' + 'margin-right' = width of containing block" /> <style type="text/css"> #wrapper { position: relative; } #div1, #div2 { border: 10px solid green; } #div1, #reference { left: 0; position: absolute; top: 0; } /* In this test, #div1, which is absolutely positioned, is #div2's containing block. In which case, the width of #div1 is shrink-to-fit to the width of #div2 plus its borders. http://www.w3.org/TR/CSS21/visudet.html#abs-non-replaced-width */ #div2 { height: 1in; margin: auto; width: 3in; } #reference { border: 20px solid red; height: 96px; width: 288px; } </style> </head> <body> <p>Test passes if there is <strong>no red</strong>.</p> <div id="wrapper"> <div id="reference"></div> <div id="div1"> <div id="div2"></div> </div> </div> </body> </html>