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As Jamie said, the calc() method seems very similar to the MS expression() rule. But besides the fact that it was vendor-specific, it’s advised not to use it because it’s a horrible performance drag.
For MSIE, the expression was evaluated every time the browser was resized and every time the mouse moved in the browser window. Avoiding the use of these epxressions made Rule #7 of Yahoo’s High Performance Web Site Rules (http://developer.yahoo.com/blogs/ydn/posts/2007/07/high_performanc_6/).
Do we know what kind of performance drawbacks -moz-calc() brings?