In a document with an external subset or external parameter entities with standalone='no', the Name given in the entity reference must match that in an entity declaration. For interoperability, valid documents should declare the entities amp, lt, gt, apos, quot. The declaration of a parameter entity must precede any reference to it. Similarly, the declaration of a general entity must precede any attribute-list declaration containing a default value with a direct or indirect reference to that general entity.

Parameter and general entity declarations must precede any references to these entities. All entity references must refer to previously declared entities. The specification also states that declaring the five predefined general entities (amp, lt, gt, apos, and quot) is a good idea. In reality, declaring the predefined general entities adds unnecessary complexity to most applications.