为什么填充没有在标签元素上工作?

I have some markup here:

我这里有一些标记:

<label>Username:</label>

<div class="input small"><div class="left"></div><div class="right"></div><div class="center">
    <input name="username" type="text" />
</div></div>

<label>Password:</label>

<div class="input small"><div class="left"></div><div class="right"></div><div class="center">
    <input name="password" type="password" />
</div></div>
<label>U