<!-- class -->
<div class$="[[foo]]"></div>
<!-- style -->
<div style$="[[background]]"></div>
<!-- href -->
<a href$="[[url]]">
<!-- label for -->
<label for$="[[bar]]"></label>
<!-- dataset -->
<div data-bar$="[[baz]]"></div>
<!-- ARIA -->
<button aria-label$="[[buttonLabel]]"></button>非polymer元素的属性值绑定：
语法：target-prop="{{hostProp::target-change-event}}"
Example:
<!-- Listens for `input` event and sets hostValue to <input>.value -->
<input value="{{hostValue::input}}">
<!-- Listens for `change` event and sets hostChecked to <input>.checked -->
<input type="checkbox" checked="{{hostChecked::change}}">
<!-- Listens for `timeupdate ` event and sets hostTime to <video>.currentTime -->
<video url="..." current-time="{{hostTime::timeupdate}}">
