exemplo attachShadow

display-date.js

(function() {

const style = `
:host {
  display: inline-block;
  margin: 0 5px;
  contain: content;
}
`
// usage <display-date format="DD/MM/YYYY"></display-date>
class DisplayDate extends HTMLElement {
  constructor() {
    super()

    const shadow = this.attachShadow({mode: 'closed'})
    shadow.innerHTML = `
  <style>${style}</style>
  <span>${date}</span>
  `
  }
}

function formatDate(format) {
  const d = new Date()
  return format
    .replace('DD', d.getDate())
    .replace('MM', (d.getMonth()+1))
    .replace('YYYY', d.getFullYear())
}

customElements.define('display-date', DisplayDate)
}).call(this)