getElementsByTagNameNS in (X)HTML documents.

Answer 1

You are likely serving the document with the wrong content type. The browser has to treat it as XML for namespaces to be recognized, so you need to use application/xhtml+xml or another XML content-type, and not text/html.

As an aside, your Doctype is wrong. If you want to use a DTD, then you will need one that includes the elements you are using from the foo namespace. If you don't, then just get rid of the Doctype — it has no bearing on rendering mode in XML documents (again, text/html documents are treated as tag soup, not XML).