I generate XHTML with Scala, which for some reason insists on writing image tags like this:
<img src="..."></img>

It is legal XML, so I can't fault Scala, but Prince complains about the </img> tag. It's not fatal, just unsightly. Is there anything I can do to turn off those errors?

If the document is truly XML, there should be no problem with writing it like that. But if the document is being parsed as HTML, then this syntax is technically invalid. Many documents claim to be XHTML, but are actually HTML, being served up with the text/html MIME type.