Are there command line or library tools for rendering webpages that use JavaScript?
web kit html to pdf works perfect, it can even produce jpg
You can look at HTMLUnit. It's main purpose is automatic web testing, but I think it may let you get the rendered page.
Well, there's the DumpRenderTree tool which is used as part of the WebKit test suites. I'm not sure how suitable it is for turning into a standalone tool, but it does what you ask for (render HTML, run JavaScript, and dump its render tree out to disk).