HTML着重元素()标记出需要用户着重阅读的内容，元素是可以嵌套的，嵌套层次越深，则其包含的内容被认定为越需要着重阅读。
元素通常被用于指示一个隐式或显式的对比。
in html 5, what was previously called block-level content is now called flow content.
结果
