HTML documents are parsed (interpreted) by a web browser according to a specific set of rules or standards. These rules are detailed in a Document Type Definition (DTD), which describes how each HTML tag is to be interpreted.
You specify the DTD to use by starting each document with a Public Identifier, such as:
For HTML 2.0, use:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
or
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0 Strict//EN">
For HTML 3.2, use:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN">
or
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2//EN">
These are only a few of the most commonly used Public identifiers. There are many other possible options.