What key feature was added in the GIF89a version?

Animation was the key feature added in GIF89a. More precisely, GIF89a introduced the Graphic Control Extension, which lets a GIF specify timing and other display behavior for individual images in a multi-image file.

The earlier GIF87a format could already store multiple images in one stream, but it did not provide the frame-delay information needed to present those images as a timed sequence. GIF89a supplied that missing control layer, allowing each image to act as a frame in an animation.

A common misconception is that GIF89a alone created modern looping GIFs. The format enabled timed animation, but looping was later popularized through Netscape's application extension, which told browsers to repeat the frame sequence. That combination produced the familiar endlessly repeating web GIF.

GIF89a also added transparency support, comments, and application-specific extension blocks. Interlacing and the basic LZW compression method were not new GIF89a features: they were already part of the broader GIF format and should not be confused with the animation-related addition.