The way tag inference is expressed in the HTML 5 spec is the problem here. What the HTML spec says is just a verbose description of SGML tag inference, by expli

The way tag inference is expressed in the HTML 5 spec is the problem here. What the HTML spec says is just a verbose description of SGML tag inference, by explicitly listing all elements that can't appear in a paragraph. Citing from [1]: Earlier [SGML] HTML DTDs included the following definition for flow capturing this in a rather straightforward way: <!ENTITY % flow "%block; | %inline;"> While HTML5 lacks it, a definition for "block content" rises again by subtracting those elements from the "flow content" category that aren't also in the "phrasing content" (inline) category. Nesting of phrasing into flow elements is about the most basic property of the HTML grammar. In SGML, the flow/phrasing hierarchy is expressed by declaring the content model of flow elements as allowing %phrasing;, where %phrasing;, like in earlier HTML DTDs, is substituted into a string such as a|abbr|... containing all phrasing elements as a name group. For example, the element declaration for the p element is as follows: <!ELEMENT p - O (#PCDATA|%phrasing;)* -(%flow_only;)> meaning that - the content of p elements can be any sequence of text content (#PCDATA) and phrasing elements, and - flow content isn't just forbidden in direct child content of p (via admitting %phrasing; elements only), but also isn't admitted anywhere in descendant content of p, as expressed by the -(%flow_only;) SGML exclusion exception, and - the end-element, but not the start-element tag of p can be omitted, as declared in ps tag omission indicator - O where the flow_only parameter entity contains block-level elements as described above, ie. flow elements that aren't also phrasing elements. [1]: http://sgmljs.net/docs/html5.html http://sgmljs.net/docs/html5.html