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Most of the login pages like Google, Outlook and Yahoo! confirm the username first and then ask for a password instead of confirming the username and password combination altogether. Isn't it less safe to go with the former practice as the intruder can guess the username first and then guess the password? whereas in the later case the intruder has to go with guessing both the options?
Also is there a website where I can find the industrial standards for the login flow?