code: function registerElement (name, elementClass, css) {
    if (customElementsStore.get(name)) {
      return
    }
  
    // store the name-clazz pair
    customElementsStore.set(name, elementClass, 'mip2')
  
    /** @type {Array<BaseElement>} */
    let customElementInstances = []
  
    loadCss(css, name)
    window.customElements.define(name, class extends BaseElement {
      static get observedAttributes () {
        return elementClass.observedAttributes
      }
  
      constructor (...args) {
        super(...args)
  
        customElementInstances.push(this)
      }
    })
  
    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 {HTMLElement[]}