文本区域水平线

textarea {
  width: 400px;
  height: 150px;
  border: 2px solid #999;
  color: #333;
  padding: 10px;
  line-height: 20px;
  background:
    repeating-linear-gradient(to bottom,
      #999 0 1px,
      transparent 1px 20px)
    0 18px / 100% content-box no-repeat;
  overflow: hidden;
}

textarea:focus {
  background: none;
}

<textarea spellcheck="false">Donec semper justo eget nisl malesuada pellentesque.

Morbi rhoncus eros sit amet magna posuere, ac imperdiet tellus finibus. Vestibulum eget varius risus. Etiam at libero ullamcorper, pharetra urna sit amet, consectetur nunc. Donec ultricies et quam quis viverra.
</textarea>