1. :optional CSS pseudo-class

    The `:optional` pseudo-class matches form inputs (`<input>`, `<textarea>`, `<select>`) which are not `:required`.

  2. Form validation

    Method of setting required fields and field types without requiring JavaScript. This includes preventing forms from being submitted when appropriate, the `checkValidity()` method as well as support for the `:invalid`, `:valid`, and `:required` CSS pseudo-classes.

  3. css selector: `:required`