b0rked.js is not a full-blown W3C-like validator.
Its purpose is to verify if an (X)HTML document or fragment is at least somewhat well-formed. It doesn't care about attributes, values, tag names, or the position of tags. It only takes a closer look at the tags. It checks if they are nested correctly and if those tags, which look like void tags, are indeed void tags.
It's intended to be used for on-the-fly validation during devlopment. This can be done by putting it between the rendering of the template(s) and the point where this new fragment is injected into the DOM.
It's meant to catch all issues which are slightly harder to spot and which happen frequently during development.
Performance is pretty good since it doesn't do all that much. On an anverage office machine, validating 100kB of markup takes less than 20 msecs.