PEs in Internal Subset |
Text from specification
In the internal DTD subset, parameter entity references can occur only where markup declarations can occur, not within markup declarations. (This does not apply to references that occur in external parameter entities or to the external subset.)
Explanation
It is only legal to use parameter entity references to build markup declarations within the external DTD subset. In other words, within the internal subset, parameter entities may only be used to include complete markup declarations.