With a new stylesheet parser compliant with CSS 2.1, one option for Internet Explorer 8 (download) would be to require filters such as the one above to be rewritten like this: -ms-filter:"progid:DXImageTransform.Microsoft.Alpha(opacity=50)";

The –ms vendor prefix explicitly marks the property as proprietary to Microsoft. By delimiting the entire value as a string, grammatical delimiter issues are avoided. Other browsers are thus able to safely ignore this declaration even if the filter value itself is syntactically invalid.

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