Hi Scott. Thanks for taking the time to evaluate this issue.
You're correct, of course, that content after the <body> is not valid HTML. Still, I'd ask you to please take a look at my tiny pull request (+4 bytes, -4 bytes). It doesn't introduce any code to the jQuery UI library that generates invalid HTML. It doesn't break the draggable library for any valid HTML documents. What's the harm, exactly?
Feel free to leave this ticket closed as notabug. I'm not claiming that there is a fault or defect in the library. I do believe, however, that pull request 987 will make jQuery UI more broadly useful. (I'm not aware of a jQuery team mandate to force HTML conformance on its users. It generally treats that as a user responsibility.)
Please take a moment to look at the pull request and respond to it either here or there. Thanks!