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Are you asking about autofill by chance that remembers the User Password ?

Private AutoFill
Safari lets you fill out forms quickly while keeping your personal information private. When you fill out a web form, websites can access the information you enter, even before you submit it. Safari prevents this by detecting web forms and presenting a drop-down field, letting you choose to use AutoFill to complete the form with information from your OS X Address Book, Outlook, or Windows Address Book. No information is added to web forms until you choose to use AutoFill.

Click on Safari upper left corner of screen while in Safari >Preference > Auto Fill > check the User Name and Password box After which, when you visit a site that you have set a password it will ask you if you wish to keep it. Not all of them, but some by my experience
 
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