The HTML DTD defines a standard HTML document type and several variations, by way of feature test entities. Feature test entities are declarations in the HTML DTD that control the inclusion or exclusion of portions of the DTD.
Certain features of the language are necessary for compatibility with widespread usage, but they may compromise the structural integrity of a document. This feature test entity selects a more prescriptive document type definition that eliminates those features. It is set to `IGNORE' by default.
For example, in order to preserve the structure of a document, an editing user agent may translate HTML documents to the recommended subset, or it may require that the documents be in the recommended subset for import.
Certain features of the language are necessary for compatibility with earlier versions of the specification, but they tend to be used and implemented inconsistently, and their use is deprecated. This feature test entity enables a document type definition that allows these features. It is set to `INCLUDE' by default.
Documents generated by translation software or editing software should not contain deprecated idioms.