It is (up to HTML 4.01 and in XHTML; not according to the latest HTML5
Working Draft). The zero-length ("empty") string is a same-document URI-
reference, see RFC 3986. (In the current HTML5 Working Draft the `action'
attribute is optional, with the default value being the zero-length string.)
Using `mailto:' there is a bad idea: Not everyone has a locally installed
mail client configured or made known to their browser. A server-side form-
mailer should be used instead.
Yes, that is to be expected. But browsers are buggy (in particular IE), and
so using a non-empty URI-reference is safer.
Sometimes, what you learn is wrong. If those wrong ideas are close to the
root of the knowledge tree you build on a particular subject, pruning the
bad branches can sometimes cause the whole tree to collapse.