a-prefetch.js

class APrefetch extends HTMLAnchorElement {
  constructor() {
    super()
  }

  connectedCallback() {
    this.addEventListener("click", event => {
      event.preventDefault()

      const xhr = new XMLHttpRequest()

      // Replace elements within the router
      xhr.addEventListener("load", function() {
        const incoming = this.responseXML.querySelector("body")
        const router = document.querySelector("body")
        router.innerHTML = incoming.innerHTML
      })

      xhr.open("GET", this.href)

      // Parse the document as XML
      xhr.responseType = "document"
      xhr.send()
    })
  }
}

window.customElements.define("a-prefetch", APrefetch, { extends: "a" })