html5 label picture usage

html5 picture label has been introduced, according to the screen resolution, let the browser choose what pictures to load.

compatibility:

usage:

<picture>
    <!-- 屏幕尺寸小于768px下 -->
    <img srcset="1.png" media="(max-width:768px)" />
    
    <!-- 屏幕尺寸大于1200px -->
    <img srcset="2.png" media="(min-width:1200px)" />
    
    <!-- 默认图片，这里可以理解为是768~1200之间的尺寸时加载的图片 -->
    <img srcset="default.png" alt="default" />
</picture>