HTML, SGML, and XML

The Standard Generalized Markup Language (SGML) was established in 1986 as an ISO standard for describing a text's semantics, rather than its appearance.
- For example, saying that "The title of this document is RFC 123", rather than "Center RFC 123 on this page with an 18pt bold font".
- SGML was championed by librarians and the publishing industry.
- The idea was to have one source document which could then be automatically transformed into many formats.
- SGML is a meta language. Over time it became very hard to write a parser for it.
The Hyper Text Markup Language (HTML) is an instance of SGML.
- It looks like:
```
    <ul>
      <li>The <b>URI</b> is defined in <a
      href="http://www.ietf.org/rfc/rfc2396.txt">RFC 2396</a>.</li>
    </ul>
```
- When Tim Berners-Lee was looking for a way to markup the text for his hypertext application he decided to base it on the well-known SGML.
- The original intention was to make all tags describe semantics (e.g., H1, STRONG, EM) but over time tags started describing display attributes (e.g., B, I, BLINK, HR).
- HTML is now at version 4.0. Many of the tags have been added by vendors. The syntax has become corrupted.
The eXtensible Markup Language (XML) is the W3C's effort to create a new meta language that is simpler than SGML.
- XML is very easy to parse. There are many parsers available for every programming language.
- XML does not define any tags. It is up to the user to define the tags that he will use. For example:
```
  <question>
    <text>Who created the first Web client and server?</text>
    <choice answer="1">Tim Berners-Lee</choice>
    <choice>Marc Andreseen</choice>
    <choice>Bill Gates</choice>
    <choice>Larry Ellison</choice>
    <choice>Al Gore</choice>
  </question>
```
- The set of tags allowed by a particular XML document are define in its DTD (Document Type Definition) or schema.
- XML is very popular.