A Bing Community thread has a Microsoft Bing representative telling SEOs that W3C complaint code can help with indexing content.
Brett Yount, the program manager at Bing Webmaster Center, said:
Clean code can help quite a bit in indexing on all the SEs. If you are just starting out, I suggest finding a W3C compliant template.
We all know that Google said time and time again that validated code does not get a ranking boost, but they never said it wouldn't help with indexing and crawling. Clean code, which can be validated through the W3C's tools, can make for more efficient crawling and possibly faster indexing.
This is at least according to Bing.
Forum discussion at Bing Community.