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The following h1 { margin: 0,0,0,0 } is invalid. The validator does not report this error but generates a NullPointerException in org.w3c.css.properties.CssMargin.setInfo because the CssMargin constructor just returns if the operator between the values is not a space. See http://www.w3.org/mid/3f3dd4c1.170929453@smtp.bjoern.hoehrmann.de
See also http://www.w3.org/Bugs/Public/show_bug.cgi?id=157