Form validity pseudo-classesThe form validity CSS pseudo-classes match <form> elements based on the constraints of a form field, such as validity (:valid, :invalid, :in-range, :out-of-range) and necessity (:optional or :required).selectorsSelectorswidely2015-07-292018-01-29https://drafts.csswg.org/selectors-4/#ui-validitycss-optional-pseudocss-in-out-of-rangehttps://developer.mozilla.org/docs/Web/CSS/Reference/Selectors/:validcss.selectors.in-rangecss.selectors.invalidcss.selectors.invalid.formcss.selectors.optionalcss.selectors.out-of-rangecss.selectors.requiredcss.selectors.validcss.selectors.valid.formhttps://nowyoucanuse.com/features--form-validity-pseudos.html