A subtle intellectual breakthrough combined the flexible power of data structures and the benign legibility of text. The result empowered both the world of publishing and the world of data sharing.
The World Wide Web began with that abstraction and for a while managed to balance being pretty with being smart. But it became a mass medium, intoxicated with good looks. HTML became hostage to the demands of graphic designers and lost its sense of abstraction. The most popular presentation technologies, like Flash, ignored data concerns and focused entirely on, well, flash.
But the pendulum is swinging back. The need for XML was felt, and it appeared. XML picked up the serious job of abstraction and freed her tarty sister HTML to become a language about how screens look.
XML now emerges as a publication format for very pure content.