old_contents: /**
   * Implement Gatsby's Browser APIs in this file.
   *
   * See: https://www.gatsbyjs.org/docs/browser-apis/
   */
  
  // You can delete this file if you're not using it
  
new_contents: /**
   * Implement Gatsby's Browser APIs in this file.
   *
   * See: https://www.gatsbyjs.org/docs/browser-apis/
   */
  
  export const onClientEntry = () => {
    return new Promise((resolve, reject) => {
      // Polyfill.io stuff
      window.__polyfillio__ = () => {
        resolve()
      }
  
      const features = []
  
      if (!("fetch" in window)) {
        features.push("fetch")
      }
  
      if (features.length) {
        const s = document.createElement("script")
        s.src = `https://cdn.polyfill.io/v3/polyfill.min.js?features=${features.join(
          ","
        )}&callback=__polyfillio__`
        s.async = true
        s.onerror = reject
        document.head.appendChild(s)
      } else {
        resolve()
      }
    })
  }
  
subject: add polyfill for IE11 to fix website