Feature summary (what you would like to be able to do and where):
Allow usage of the filter CSS property on pages with a content model other than sanitized-css. Specifically, in the style attribute of elements.
Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):
I tried to make the Ditto sprite in a userbox at bulba:User:Bfdifan2006 yellow by wrapping it with a div whose filter is hue-rotate(135deg), but MediaWiki blocked the property stating it's insecure (by changing it to /* insecure input */. I actually commented it out in the wikitext.
Benefits (why should this be implemented?):
This could come useful for wikis that are a bit too reluctant to adopt TemplateStyles, and also for users that are a bit confused about the extension's functionality.
This was blocked in 2010 for security reasons since at that time, Internet Explorer had its own insecure version of filter that was unrelated to the later W3C property (more information), but the IE property was deprecated in version 9 and removed from the core in version 10. The browser is going to lose Microsoft support soon due to the success of the Blink-based Microsoft Edge.
It was not a W3C-compatible property, it used a different syntax. However, the W3C-compatible property is safe.
A showcase of this property failing can be found here.