CSS expressions degrade rendering performance; replacing them with alternatives will improve browser rendering for IE users.
Page Speed recommends:
Use standard CSS properties if possible.
IE 8 is fully CSS-standards-compliant; it supports CSS expressions only if run in "compatibility" mode, but it does not support them in "standards" mode. If you do not need to maintain backwards compatibility with older versions of IE, you should convert any instances of expressions used in place of standard CSS properties to their CSS standard counterparts. For a complete list of CSS properties and IE versions that support them, see the MSDN CSS Attributes Index. If you do need to support older versions of IE in which the desired CSS properties are not available, use JavaScript to achieve the equivalent functionality.
Use JavaScript to script styles.
If you are using CSS expressions for dynamic styling, it makes sense to rewrite them as pure JavaScript to both improve performance in IE and get the benefit of supporting the same functionality in other browsers at the same time.
Click "Read more" to view an example.
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