How do i embed Orbeon form builder in a non java webapp, (Plain Javascript) as well as customising its look (CSS). I am currently embedding it using an iframe but it doesn't seem to be a very good option. Any suggestions are appreciated.
Embedding Form Builder with the JavaScript embedding API isn't supported yet. At this point, you can only embed Form Builder with the Java embedding API.
There is no fondamental reason why this couldn't possible from the JavaScript API; it is just a question of issues that still need to be ironed out (for more on this, see issue #4483). So, as I write this, if you can't use the Java embedding API, you're correct and the only way to do this is to use an iframe.