Watching a page loadAn article can stand on its own; a section groups a topic within it.文章可以独立成立；小节在文章里归拢一个主题。 What we observedThe browser reads a document tree. Heading levels help people navigate that tree.浏览器读取的是一棵文档树。标题级别帮助人们在这棵树里导航。