I’d noticed there were some sites still showing up with missing pictures or bad formatting in the preview. I spent some time debugging some of the problem cases, and realized there were some bugs in the code I was using to insert the tag that resolves relative URLs. I put in fixes for those problems, which made a lot of CSS based sites appear much better.
It also allowed some scripts to run that hadn’t before, so I then had to fix the bugs in my script blocking that let those through. The end result is that preview works a lot more reliably.