此处input为什么要添加span父级呢?有什么好处么?
<div class="sign-input">
<span>帐 号:</span>
<span><input autocomplete="on" name="u" id="u" type="text"
style="ime-mode: disabled" class="input01" tabindex="1" value="QQ号码或Email
帐号" onFocus="if (value =='QQ号码或Email帐号'){value =''}"
onBlur="if (value ==''){value='QQ号码或Email帐号';}" /></span>
</div>