mindmap
    ((HTML))
        (Features)
            (不区分大小写)
            (主要组成成分)
                (Opening tag)
                (Closing tag)
                (Content)
                (Element)
            (元素类型)
                (块级元素)
                (内联元素)
            (元素内容类型 html5 https://html.spec.whatwg.org/multipage/indices.html#element-content-categories)