Totally unsafe, as if a user makes an error, fills in incorrect data, he or she may have problem re-checking the whole form. In this case it is very simple. But even for login forms it is important. For example - in some systems (e.g. Pocket or Wunderlist on my iPhone) I keep filling in my email address as login and then being informed that the credentials are wrong. After a minute of fighting with the password, I start realizing that maybe the login is incorrect. But wait, the email address is ok! Takes me a while and needs deleting the whole email address for the label to reappear, telling me that I should use a login rather than email address.