The :valid CSS pseudo-class represents any <input> or <form> element whose content validates correctly according to the input's type setting. This allows to easily make valid fields adopt an appearance that helps the user confirm that their data is formatted properly.
The :valid selector allows you to select < input > elements that contain valid content, as determined by its type attribute. :valid is defined in the CSS Selectors Level 3 spec as a “validity pseudo-selector”, meaning it is used to style interactive elements based on an evaluation of user input.
The :valid selector selects form elements with a value that validates according to the element's settings.
This selector has one particular use: providing a user with feedback while they are interacting with a form on the page. The example below uses CSS to turn the “Email” fields red or green, responding to the whether or not the contents match a valid email address pattern: