"Form validation has been a finicky business since the web was born. First came the server-side validation error summary. Then we evolved to client-side validation to verify results inline. Now, we have the marching giant that is HTML5 and CSS3: HTML5’s forms chapter offers new input types and attributes that make validation constraints possible. CSS3’s basic UI module provides several pseudo-classes to help us style those validation states and change a field’s appearance based on the user’s actions.