Consider adding an item to address validation of input elements

[JanMiksovsky]

Generally speaking, it's not clear how people should create web components as form input elements. But as that gets resolved, we may want to add checklist items to cover such components.

As an example, components that are intended for use in forms should probably take advantage of validation APIs.

• https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/HTML5/Constraint_validation
• http://www.w3.org/TR/wai-aria/states_and_properties#aria-invalid

[Hat tip to Alice Boxhall and Rob Dodson for suggesting this item.]