MySpace

 MySpace  was a popular social networking site in the 2000s that accidentally allowed HTML and CSS in profile fields, unintentionally introducing millions to the ability to customize their profiles with old independent-web-like graphics, animations, and music.

MySpace relaunched in 2013 as a music listening service with social aspects.

How to
From a historical perspective:
 * 2006-04-18 Hacking A More Tasteful MySpace

Articles
In chronological order, note the stark changes in headlines:
 * 2006-07-11 Mashable: MySpace, America's Number One "MySpace is also winning market share from other social networks - in June, it captured 80% of social networking traffic, up from 76% in April. Facebook is second with 7.6%"
 * 2007-02-08 The Guardian: Will MySpace ever lose its monopoly?
 * 2018-04-11 TheOutline.com: MySpace Tom beat Facebook in the long run
 * 2020-02-14 MySpace and the Coding Legacy it Left Behind "… while rewriting MySpace, Harriman and Nguyen made a major mistake—they failed to block users from adding their own HTML and CSS to their profile pages. In Angwin’s telling, the team’s mistake was what “allowed users to build colorful backgrounds and wallpaper and load them onto their MySpace pages."