Form validation

Relevant features? Maybe create new issues for each

## `HTMLInputElement`

### Methods

- [ ] [checkValidity](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/checkValidity)
- [ ] [reportValidity](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/reportValidity)
- [ ] [checkCustomValidity](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/setCustomValidity)

### Attributes

- [ ] [validity](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/validity)
- [ ] [willValidate](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/willValidate)
- [ ] [validationMessage](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/validationMessage)

### Events

- [ ] [Invalid](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/invalid_event)

## `HTMLFormElement`

- [ ] If validation fails, form will not be submitted, and there shall be no submit event.

Start with the linked MDN documentation for HTMLInputElement validation methods, attributes, and the invalid event, then review the repository's existing HTMLInputElement and HTMLFormElement entry points. Done means the listed validation APIs are supported and failed form validation prevents submission without dispatching a submit event.

Written by the indexing model from the issue text.