Five seconds before every kill, kept on the account and replayed in the browser.
Every kill on the world writes a killcam: the five seconds before it, every tile, animation and hitsplat, packed on the server rather than reconstructed after the fact. The replay runs in 3D on the site, and the day's best one loops on a stage at home.
Nothing about it is a highlight reel. The record is the fight as the server saw it, which is also what the battle log, the hiscores and the kill feed read from.