### Synchronous Custom Validator

Extend `Vaadin.TextFieldElement` to create your own custom element, then override the `checkValidity()` method to validate the user's input

### Asynchronous Custom Validator

Extend `Vaadin.TextFieldElement` to create your own custom element, then override the `checkValidity()` to make your asynchronous validation, and finally set the `invalid` flag appropriately in the callback.