code: function setRootFont (width, viewportWidth, force) {
    const doc = window.document
    const rem = width * 750 / viewportWidth / 10
    if (!doc.documentElement) { return }
    const rootFontSize = doc.documentElement.style.fontSize
    if (!rootFontSize || force) {
      doc.documentElement.style.fontSize = rem + 'px'
    }
    info.rem = rem
    info.rootValue = viewportWidth / 10
  }
docs: set root font-size for rem units. If already been set, just skip this.