1. 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.

  2. javascript classes: private class fields

  3. javascript classes: private class fields 'in'

  4. javascript classes: public class fields

  5. javascript classes: static class fields