完整的代码如下 - 在线编辑器

          <style>
  .search {
    width: 130px; /* 设定起始宽度 */
    box-sizing: border-box;
    border: 2px solid #ccc;
    border-radius: 4px;
    font-size: 16px;
    background-color: white;
    background-image: url('//u.mifengjc.com/images/search-icon.png');
    background-position: 10px 10px;
    background-repeat: no-repeat;
    padding: 12px 20px 12px 40px;
    -webkit-transition: width 0.4s ease-in-out;
    transition: width 0.4s ease-in-out; /* 设置宽度改变时的动画 */
  }

  .search:focus {
    width: 100%;
  }
</style>

<p>动画搜索表单:</p>

<form>
  <input class="search" type="text" name="search" placeholder="搜索...">
</form>

        