What application-specific block made looping animated GIFs common in browsers?

The application-specific block that made looping animated GIFs common in browsers was the Netscape Application Block.

GIF89a could store multiple images and frame delays, so it already supported the ingredients of animation. However, a normal animated GIF stopped after its final frame. Netscape used the format’s Application Extension mechanism to add looping instructions without changing the basic GIF structure.

The block is identifiable in files by the application identifier “NETSCAPE2.0.” Its repetition field specifies how many times the animation should play; a value of zero means repeat indefinitely. Although this extension was not formally part of the original GIF89a standard, other browsers adopted it, making endlessly looping GIFs broadly interoperable.

A common mix-up is confusing the Netscape Application Block with the Graphic Control Extension. The Graphic Control Extension controls per-frame details such as delay and transparency; the Netscape block controls repetition. GIF itself was released by CompuServe in 1987, while the looping convention became widespread after Netscape added support in the mid-1990s.