ブラウザがWebPをサポートしているかどうか

canUseWebP.js

export default function() {
  var elem = document.createElement('canvas')

  if (!!(elem.getContext && elem.getContext('2d'))) {
    // was able or not to get WebP representation
    return elem.toDataURL('image/webp').indexOf('data:image/webp') == 0
  }

  // very old browser like IE 8, canvas not supported
  return false
}