In computing, the word “document” is unfortunately overloaded. It used to mean text document: a unit of structured human-readable text. But now we talk about “JSON documents” and “YAML documents”. Document databases are not text repositories but databases where the records are JSON blobs.

“Markup” is similarly confused. YAML used to mean Yet Another Markup Language. But it’s not a markup language, it’s a data exchange language.¹

XML is widely derided by programmers because it’s verbose—when used a data exchange language, where the tag-content ratios are very poor², e.g.:

<star>
  <bayerName>Beta Pictoris</bayerName>
  <spectralType>A6V</spectralType>
  <mass>1.75</mass>
  <luminosity>8.7</luminosity>
  <position>
    <rightAscension>05h 47m 17.1s</rightAscension>
    <declination>−51°03′59″</declination>
    <distance>63.4ly</distance>
  </position>
  <age>
    <min>20My</min>
    <max>24My</max>
  </age>
</star>

But as a markup language XML is much more tolerable:

<document>
  <title>A UNIVERSAL DECLARATION OF HUMAN RIGHTS</title>
  <h1>Preamble</h1>
  <p>
    Whereas recognition of the inherent dignity and of the equal and inalienable
    rights of all members of the human family is the foundation of freedom,
    justice and peace in the world,
  </p>
  <p>
    Whereas disregard and contempt for human rights have resulted in barbarous
    acts which have outraged the conscience of mankind, and the advent of a
    world in which human beings shall enjoy freedom of speech and belief and
    freedom from fear and want has been proclaimed as the highest aspiration of
    the common people,
  </p>
</document>

XML is precisely what it says on the tin: an extensible markup language. It’s a markup language with a completely uniform syntax so that the alphabet of markup elements is customizable. And for what it is, there is truly no replacement. Every other markup language supports only a limited set of markup directives defined from the factory. The tradeoff is generality for ease of authoring: limited markup languages can have terser syntax for specific elements.

So why did XML come to be used as a data exchange language? Partly because, despite its roots in SGML (the Common Lisp of markup languages), the creators advertised it as a general format to exchange any digital information.

But also because it’s a fairly natural evolution. You start out writing your XML documents using a generic alphabet with things like header, p, table, etc. Then after writing enough documents of the same type, you start to notice common patterns, and decide to factor out those patterns by making a new schema with domain-specific elements, and an XSLT stylesheet to compile that down to the generic alphabet. And now you’ve turned your XML documents into data blobs, to be compiled into documents.

Footnotes