However under HTML 4 the current specification says that an empty action attribute resolves to the document's base URL (likewise for other attribute values that are supposed to be interpreted as URLs). Unless otherwise stated the document's base URL is the URL of the document.
HTML5 explicitly rules out an empty value - even though the empty string is a valid URL. Still haven't found out why that decision was made.
According to the discussion of the attribute's value, leaving it out completely has the desired effect.
In other words, in HTML5, the default value for a form's action attribute is one that cannot be specified explicitly!
(Apparently, [font=monospace]action="."[/font] also works, but as far as I can make out, by rights it should , given a base of [font=monospace]http://www.example.com/a/b/form.php[/font], resolve into [font=monospace]http://www.example.com/a/b/[/font] and that's not what you want....)