The validationMessage read-only property of the HTMLObjectElement interface returns a DOMString representing a localized message that describes the validation constraints that the control does not satisfy (if any). This is the empty string if the control is not a candidate for constraint validation (willValidate is false), or it satisfies its constraints.

The willValidate read-only property of the HTMLObjectElement interface returns a boolean that indicates whether the element is a candidate for constraint validation.

Returns true if the element's value has no validity problems; false otherwise. Fires an invalid event at the element in the latter case.

Returns true if the element's value has no validity problems; otherwise, returns false, fires an invalid event at the element, and (if the event isn't canceled) reports the problem to the user.

Sets a custom error, so that the element would fail to validate. The given message is the message to be shown to the user when reporting the problem to the user. If the argument is the empty string, clears the custom error.

The invalid event fires when a submittable element has been checked for validity and doesn't satisfy its constraints.

The :enabled CSS pseudo-class represents any enabled element. An element is enabled if it can be activated (selected, clicked on, typed into, etc.) or accept focus. The element also has a disabled state, in which it can't be activated or accept focus.

The :disabled CSS pseudo-class represents any disabled element. An element is disabled if it can't be activated (selected, clicked on, typed into, etc.) or accept focus. The element also has an enabled state, in which it can be activated or accept focus.

The :valid CSS pseudo-class represents an element whose contents validate successfully. This allows to easily make valid fields adopt an appearance that helps the user confirm that their data is formatted properly.

The :invalid pseudo-class must match any element falling into one of the following categories:

• elements that are candidates for constraint validation but that do not satisfy their constraints
• form elements that are the form owner of one or more elements that themselves are candidates for constraint validation but do not satisfy their constraints
• fieldset elements that have of one or more descendant elements that themselves are candidates for constraint validation but do not satisfy their constraints

To turn a custom element into a form-associated custom element requires a few extra steps:

• Add a static formAssociated property to your custom element class. This tells the browser to treat the element like a form control.
• Call the attachInternals() method on the element to get access to extra methods and properties for form controls, like setFormValue() and setValidity().
• Add the common properties and methods supported by form controls.

Called when the browser associates the element with a form element, or disassociates the element from a form element

Called after the disabled state of the element changes, either because the disabled attribute of this element was added or removed; or because the disabled state changed on a >fieldset< that's an ancestor of this element. The disabled parameter represents the new disabled state of the element.

Called after the form is reset.

Called when:

• The browser restores the state of the element (for example, after a navigation, or when the browser restarts). The mode argument is "restore" in this case.
• Called when the browser's input-assist features such as form autofilling sets a value. The mode argument is "autocomplete" in this case.

• https://web.dev/more-capable-form-controls/
• https://developer.mozilla.org/
• https://html.spec.whatwg.org/