static get styles() {
    return [
      css`
        .accordion {
          display: flex;
          flex-direction: column;
        }

        .accordion ::slotted(.invoker) {
          margin: 0;
        }

        .accordion ::slotted(.invoker)[expanded] {
          font-weight: bold;
        }

        .accordion ::slotted(.content) {
          margin: 0;
          visibility: hidden;
          display: none;
        }

        .accordion ::slotted(.content[expanded]) {
          visibility: visible;
          display: block;
        }
      `,
    ];
  }
exclusive property allows only one accordion slot to be open at a time