<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel='stylesheet prefetch' href='https://res.wx.qq.com/open/libs/weui/0.3.0/weui.css'>
</head>
<body>
<div class="weui_cells_title">意见反馈</div>
<div class="weui_cells weui_cells_form">
        <div class="weui_cell_bd weui_cell_primary">
        <textarea class="weui_textarea" id="textarea" placeholder="请输入意见反馈" rows="3"></textarea>
        <div class="weui_textarea_counter"><span id="count">0</span>/200</div>
        </div>
</div>
<script src="//cdn.bootcss.com/jquery/3.1.0/jquery.min.js"></script>
<script>
$(function(){
  var max = 200;
  $('#textarea').on('input', function(){
     var text = $(this).val();
     var len = text.length;
     $('#count').text(len);
     if(len > max){
       $(this).closest('.weui_cell').addClass('weui_cell_warn');
     }
     else{
       $(this).closest('.weui_cell').removeClass('weui_cell_warn');
     }
  });
})
</script>
</body>
</html>

完整方案，需要引用jquery。

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