HTMLFieldSetElement: setCustomValidity() method

The setCustomValidity() method of the HTMLFieldSetElement interface sets the custom validity message for the <fieldset> element. Use the empty string to indicate that the element does not have a custom validity error.

The <fieldset> element is not a candidate for constraint validation. The reportValidity() method will not cause the custom error message to be displayed to the user, but does set the customError property of the element's ValidityState object to true and the valid property to false.

setCustomValidity(string)

const errorFieldSet = document.getElementById("checkErrors");
const errors = issuesToReport();
if (errors) {
  errorFieldSet.setCustomValidity("There is an error");
} else {
  errorFieldSet.setCustomValidity("");
}