<script>

  class XControl extends HTMLElement {

    static formAssociated = true;

    constructor() {

      super();

      this.internals = this.attachInternals();

      this.internals.setValidity({valueMissing: true, customError: false}, 'Value missing');

      // Expecting "customError: false"

      alert(`customError: ${this.internals.validity.customError}`);

    }

  }

  customElements.define('x-control', XControl);

</script>

<x-control></x-control>