Common CSS Mistakes

• When specifying one of the "background-*" properties, especially background-image, be careful that you don't include values for any of the other "background-*" properties, e.g. background-position and background-repeat, along with the proper value. Either use the shorthand "background" property or specify the properties separately.
• Missing units. There are several units of linear measurement in CSS, one of the most common is pixels (px) which is the same unit of measurement that HTML attributes use.
• The z-index property only applies to elements with a 'position' property set to something other than 'static', which is the default.
• The top, left, bottom, and right properties only apply to relatively or absolutely positioned elements. ("position:fixed" is considered a form of absolute positioning.)
• :link, :visited, :hover, :focus, :active should be used in that order. (I, for one, very rarely use the ":link" pseudo-class.)
• position:relative moves the element from where it would normally render and leaves the space that it would normally occupy there.
• Forgetting to always specify a background color for a page. The background is usually white by default, but you can not rely on that.
• Guidelines about how to use CSS with XHTML and XML properly.
• Relative positioning should only be used to slightly adjust the position of elements, not to layout a page. Use floats, margins and, when needed, absolute positioning instead.
• Don't overuse absolute positioning.
• Placing @import rules where they aren't allowed. "CSS 2.1 user agents must ignore any '@import' rule that occurs inside a block or after any valid rule other than an @charset or an @import rule."