message: render Emotion styles statically
diff: import React from 'react'
  import Document, {Head, Main, NextScript} from 'next/document'
  +import {extractCritical} from 'emotion-server'
  export default class MyDocument extends Document {
  + static getInitialProps({renderPage}) {
  + const page = renderPage()
  + const {css} = extractCritical(page.html)
  + return {...page, css}
  + }
  +
  render() {
  const {
  + css,
  // the assetPrefix is set in next.config.js
  __NEXT_DATA__: {assetPrefix = ''}
  } = this.props
  @@ -19,6 +27,8 @@ export default class MyDocument extends Document {
  <link rel="apple-touch-icon" href={asset('/static/assets/apple-touch-icon.png')} />
  {/* See: https://github.com/zeit/next-plugins/tree/master/packages/next-sass#usage */}
  <link rel="stylesheet" href={asset('/_next/static/style.css')} />
  + {/* eslint-disable-next-line react/no-danger */}
  + <style data-emotion dangerouslySetInnerHTML={{__html: css}} />
  <meta name="viewport" content="width=device-width,initial-scale=1" />
  <meta name="og:title" content="Primer React" />
  <meta name="description" content="Primer components built with React.js." />
  