See also:
The Makefile iterates over all the HTML documents in this directory and several subdirectories, invoking sgmls on each (after setting SGML_PATH to find the dtd etc.)
The make rule may actually try each document twice: once with doctype.sgml prepended, and once solo. The doctype.sgml file has a <!DOCTYPE ... declaration that's missing from most HTML documents.
The "nominal" directory contains several of the test documents cleaned up to remove obsolete features.