invalid (:invalid)

The :invalid CSS pseudo-class represents any <input> or <form> element whose content fails to validate according to the input’s type setting. This allows you to easily have invalid fields adopt an appearance that helps the user identify and correct errors.
 
By default, Gecko does not apply a style to the :invalid pseudo-class. However it does apply a style (a red “glow” using the box-shadow property) to the :-moz-ui-invalid pseudo-class, which applies in a subset of cases for :invalid.
 
You can disable the glow using the following CSS, or completely override it to alter the appearance of invalid fields.

Example

<!DOCTYPE html>
<html>
    <head>
        <style>
            input:invalid {
                background-color: #ffdddd;
            }
            
            input:valid {
                background-color: #ddffdd;
            }
            
            input:required {
                border-color: #800000;
                border-width: 3px;
            }
        </style>
    </head>
    <body>
        <form>
            <label>
                Enter a URL:
            </label>
            <input type="url" />
            <br/>
            <br/>
            <label>
                Enter an email address:
            </label>
            <input type="email" required/>
        </form>
    </body>
</html>