PageSpeed: Avoid CSS expressions (deprecated)
CSS expressions degrade rendering performance; replacing them with alternatives will improve browser rendering for IE users.
Note: This rule was removed in Page Speed 1.8 since CSS expressions are no longer supported in any modern browsers (IE8 in standards mode or higher).
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Details from Google
- To emulate standard CSS properties supported by other browsers but not yet implemented by IE.
Unfortunately, the performance penalty imposed by CSS expressions is considerable, as the browser reevaluates each expression whenever any event is triggered, such as a window resize, a mouse movement and so on. The poor performance of CSS expressions is one of the reasons they are now deprecated in IE 8. If you have used CSS expressions in your pages, you should make every effort to remove them and use other methods to achieve the same functionality.