rowan-m

// class InputKnob
_drawState() {
  this._container.style.setProperty('--angle', `${this.value}rad`);
}

rowan-m

class InputKnob extends HTMLElement {
  constructor() {
    super();
    this.attachShadow({ mode: 'open' });
    this.shadowRoot.appendChild(template.content.cloneNode(true));
    this._container = this.shadowRoot.getElementById('container');
  } // ✂️ rest of the class omitted
}

rowan-m

const template = document.createElement('template');
template.innerHTML = `
  <style>
    #container {
      --angle: 0rad;
      transform: rotate(var(--angle));
    }
  </style>
  <div id="container" part="container">
    <slot></slot>

rowan-m

input-knob::part(container) {
  display: block;
  background: #cadbbc;
  border: 1rem dashed #356211;
  box-shadow: 0.3rem 0.3rem 0.3rem rgba(0, 0, 0, 0.5);
  width: 8rem;
  height: 8rem;
}

rowan-m

<style>
  #container {
    --angle: 0rad;
    transform: rotate(var(--angle));
  }
</style>
<div id="container" part="container">
  <slot></slot>
</div>

rowan-m

{
  --angle: 0rad;
  transform: rotate(var(--angle));
}

rowan-m

input-knob {
  display: block;
  background: #cadbbc;
  border: 1rem dashed #356211;
  box-shadow: 0.3rem 0.3rem 0.3rem rgba(0, 0, 0, 0.5);
  width: 8rem;
  height: 8rem;
}

rowan-m

class InputKnob extends HTMLElement {
  constructor() {
    // Nothing to do in here at the moment, but we do need to call the parent
    // constructor
    super();
  }
  // Watch for changes on the 'value' attribute
  static get observedAttributes() {
    return ['value'];
  }

rowan-m

const knob = document.querySelector('input-knob');
knob.value = 2;

rowan-m

<input-knob value="1"></input-knob>