Good points, some of which can be addressed: Re: conditionals SGML allows marked sections to have parameter entities as keywords, and recognizes INCLUDE/IGNORE

Good points, some of which can be addressed: Re: conditionals SGML allows marked sections to have parameter entities as keywords, and recognizes INCLUDE/IGNORE in their expansions: <!entity % e "include"> ... <![ %e [ will only be included if e expands to INCLUDE ]]> Re: mixed user/system text SGML supports quite restrictive content model exclusions, eg the "-(script)" part below makes SGML reject script elements in main's content anywhere: <!element main - - ANY -(script)> Moreover, you can declare an entity as a so-called data text entity, special chars in which are preserved Re: running legacy parts There's no difference to what PHP does now eg just running through the page/template doc until the end, erring out on validation errors?