Custom elements with bel (via https://github.com/shama/bel/compare/master...timwis:custom-elements?expand=1)

index.js

const html = require('bel')

window.CustomButton = function (props) {
  console.log('Called CustomButton', props)
  return html`
    <button style="background-color: ${props.color}">
      ${props.label}
    </button>
  `
}

const tree = html`
  <div>
    <CustomButton color="red" label="Add" />
  </div>
`

document.body.appendChild(tree)