[JimDabell]

You can’t. At minimum you need to call customElements.define().

[throwitaway1123]

You absolutely can. It's the primary purpose of the DSD (Declarative Shadow DOM), one of the many new specifications people complain incessantly about.

  <my-component>
    <template shadowrootmode="open">
      <style>
        ::slotted(*) {
          font-weight: bold;
          font-family: sans-serif;
        }
      </style>
      <slot></slot>
    </template>
    <p>content</p>
  </my-component>