The preliminary CSS3 Generated & Replaced Content Module spec suggests a very useful mechanism for providing content fallback in cases where the first content is unusable. I believe implementing this should be a high priority among CSS-related work as it can eliminate current JavaScript and Flash means of image replacement.
I am voting for this, as I find it extremely important for the styling of XML documents. The attr(href,url,fallback) construct would make it possible for the first time to show images referenced in arbitrary XML documents.
<rdar://problem/96849441>