HTMLButtonElement: validationMessage property

The validationMessage read-only property of the HTMLButtonElement interface returns a string representing a localized message that describes the validation constraints that the <button> control does not satisfy (if any). This is the empty string if the control is not a candidate for constraint validation (the <button>'s type is button or reset), or it satisfies its constraints.

If the <button> is a candidate for constraint validation (the type is set or defaults to submit and HTMLButtonElement.willValidate is true) and the constraints are not met (there is a non-null ValidityState.customError), the value is the error message that would be shown to the user during element validation.