延迟加载JS

Labjs库解决异步加载的兼容性问题

<script src="a.js" defer="defer"></script>
<script src="b.js" defer="defer"></script>
<script src="c.js" defer="defer"></script>

<script src="a.js" async="async"></script>
<script src="b.js" async="async"></script>
<script src="c.js" async="async"></script>