HTML Attribute Autocomplete

Does what you would expect: Assists software like password managers to automatically fill in form fields.
The attribute is available on <input> elements that take a text or numeric value as input, <textarea> elements, <select> elements, and <form> elements.

What's interesting, is that they also serve a purpose on hidden form attributes. When applied to a hidden input that has a value set, the user client can use these values for context. For example to choose the most relevant credit card based on the amount and currency:

<form method=post action="step2.cgi">
  <input type=hidden autocomplete=transaction-currency value="CHF">
  <input type=hidden autocomplete=transaction-amount value="15.00">
  
  <input type=text inputmode=numeric autocomplete=cc-number>
  <input type=month autocomplete=cc-exp>
  
  <input type=submit value="Continue...">
</form>

Important / interesting field Types

• name -> Full Name
• username
• new-password -> When registering/changing password
• current-password -> Password belonging to username
• one-time-code
• street-address
• country
• postal-code
• cc-* -> credit card info (e.g. cc-name, cc-number...)
• bday -> Birthday
• tel
• email

Sources:
HTML attribute: autocomplete
HTML Standard