style-preload.js

var sp = [].slice.apply(document.querySelectorAll('link[rel=preload][as=style]'))

sp.forEach(function(item) {
  item.onload = function() {
    item.rel = 'stylesheet'
    item.onload = null
  }
})

/**
 * For browsers that do not support rel=preload
 */
function rs() {
  sp.forEach(function(item) {
    item.rel = 'stylesheet'
  })

  window.removeEventListener('load', rs)
}
window.addEventListener('load', rs)