Form validation helps us to ensure that users fill out forms in the correct format, making sure that submitted data will work successfully with our applications.
Note that ARIA is an independent specification that's not specifically related to HTML5.
In my final example, we built something that makes use of Ajax to validate at the server.
The form will ask you to name a new product and will then check to ensure that the name is unique.
When an element is valid, the following things are true: Adds a custom error message to the element; if you set a custom error message, the element is considered to be invalid, and the specified error is displayed.
This lets you use Java Script code to establish a validation failure other than those offered by the standard constraint validation API.