One thing to think about here is that one aspect of web/password security is to not let any information out about the user database table via public-facing forms. In this case, an HTML form that switches buttons or form-actions upon seeing an email in the user table can pose a security hole.
This is also why you most likely see vague "invalid email address or password" dialogue when you enter a bad password on a site you know you have a login with.
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One thing to think about here is that one aspect of web/password security is to not let any information out about the user database table via public-facing forms. In this case, an HTML form that switches buttons or form-actions upon seeing an email in the user table can pose a security hole.
This is also why you most likely see vague "invalid email address or password" dialogue when you enter a bad password on a site you know you have a login with.