CSS - 为什么我不能在我的身高上应用％？

Question

我正在尝试将％应用于我的身高，但我不能。相反，div只接受px，有人知道我做错了什么？

.left-bar{
        height:200px;

//如果我使用px工作正常，但如果我使用％不起作用

的CSS：

html,
body {
  height: 100%;
  width: 100%;
  overflow-y: hidden;
  border:1px dashed gray;
}

.left-bar{
    height:200px; // why I cannot use % for my height?
    width:30%;
    border:1px solid green;
    overflow-y:scroll;
}

HTML：

    <div class="container-fluid">

        <div class="md-col-12">
        header
        </div>

    <div class="col-md-12">
    <div class="left-bar">
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    </div>

    </div>



    <div class="col-md-12">
    footer
    </div>
    </div>

jsfiddle：https://jsfiddle.net/q7kjm9pn/1/

我试过这样做： Percentage Height HTML 5/CSS

不重复 不重复见： 编辑：https://jsfiddle.net/q7kjm9pn/5/