<!DOCTYPE html> <!-- Nested fieldsets should be assigned validation correctly --> <html> <head> <style> fieldset:valid { background-color: green; } fieldset:invalid { background-color: red; } </style> </head> <body> <fieldset style="background-color:red;"> <fieldset style="background-color: green;"> <input> </fieldset> <fieldset style="background-color: red"> <input required> </fieldset> </fieldset> </body> </html>