code: function getPropName (prop) {
      let res
      if (testEl.style[prop] !== undefined) {
        res = prop
      } else {
        for (let key in vendors) {
          let val = '-' + vendors[key] + '-'
          if (testEl.style[val + prop] !== undefined) {
            res = val + prop
            break
          }
        }
      }
  
      return res
    }
docs: 获取正确的css属性名
  
  @param {string} prop 属性名
  @return {string | undefined} 自动添加前缀后的属性名或者undefined