Client-side form: Documentation for field strategy

[haijian-vaadin]

As a developer, when validation fails, I might want to customize the way of displaying validation errors. Vaadin form input components work out-of-the-box, but if I want to have a custom behavior or use non-Vaadin components, I need to how to use a custom field strategy to do that.

DoD:
Have an article under the Creating Client-side Forms section about the field strategy.

Explain

• What is field strategy?
• How to define a custom strategy?
• What properties/attributes/styles are set when validation fails? Give an example using the properties/attributes/styles for a Vaadin component and a non-Vaadin component.