Si le echas un vistazo a la especificación... verás que pone cláramente: "It cannot contain block-level elements<http://www.w3.org/TR/REC-html40/struct/global.html#block-inline>(including P <http://www.w3.org/TR/REC-html40/struct/text.html#edef-P> itself)." http://www.w3.org/TR/REC-html40/struct/text.html#edef-P