The constraint validation API adds the following properties / methods to DOM nodes.The property of a DOM node contains the message the browser displays to the user when a node's validity is checked and fails.
To help ease the burden on developers, HTML5 introduced a concept known as constraint validation - a native means of implementing client side validation on web forms.
In this tutorial I will: Per the spec how errors are presented to the user is left up to the browser itself.
However, the spec does provide a full DOM API, new HTML attributes, and CSS hooks authors can use to customize the experience.
The following example shows how the validation rules for the Most validators have default error messages that will be added to the model being validated when its attributes fail the validation.
For example, the required validator will add a message "Username cannot be blank." to a model when the Some validators may support additional error messages to more precisely describe different causes of validation failures.