高性能异步图片加载器的JS库：lazysizes

<script src="lazysizes.min.js" async=""></script>
使用示例
<!-- non-responsive: -->
<img data-src="image.jpg" class="lazyload" />
<!-- responsive example with automatic sizes calculation: -->
<img
    data-sizes="auto"
    data-src="image2.jpg"
    data-srcset="image1.jpg 300w,
    image2.jpg 600w,
    image3.jpg 900w" class="lazyload" />
<!-- iframe example -->
<iframe frameborder="0"
    class="lazyload"
    allowfullscreen=""
    data-src="//www.youtube.com/embed/ZfV-aYdU4uE">
</iframe>