code: function registerElement (name, elementClass, css) {
    if (customElementsStore.get(name)) {
      return
    }
  
    // store the name-clazz pair
    customElementsStore.set(name, elementClass, 'mip1')
  
    /** @type {Array<BaseElement>} */
    let customElementInstances = []
  
    // Override createdCallback to count element instances
    let mipElementProto = createMipElementProto(name)
    let createdCallback = mipElementProto.createdCallback
    mipElementProto.createdCallback = function () {
      createdCallback.call(this)
      customElementInstances.push(this)
    }
  
    loadCss(css, name)
    document.registerElement(name, {
      prototype: mipElementProto
    })
  
    return customElementInstances
  }
docs: Register MIPElement.
  
  @param {string} name Name of a MIPElement.
  @param {Class} elementClass element class
  @param {string} css The csstext of the MIPElement.
  @return {Array<HTMLElement>|undefined}