I made a brief check on Chrome and Firefox (linux). That technique goes with the "name" attribute, rather than with the "id". I wasn't sure about that because we always put both with the same value.
In theory, you could of course have multiple forms called with the same name, but in practice this shouldn't happen, at least in my opinion. Field names can recurr often, but formName + fieldName combined are usually unique in a page, so that dot notation should be safe enough.
