Описание тега html5-validation
HTML5 Validation refers to the constraint-validation engine in HTML5 which allows easy validation of forms.
HTML5 Validation allows developers to easily perform client-side validation of user input on forms. Previously, client-side validation had to be performed by custom Javascript code prior to form submission. Using HTML5 Validation, developers can attach validation constraints directly to the input element, and these elements will be validated prior to form submission.