Interlaced GIFs. Early browsers only rendered the single lines as they came in (leaving the unloaded parts blank), but eventually browsers started doubling the

Interlaced GIFs. Early browsers only rendered the single lines as they came in (leaving the unloaded parts blank), but eventually browsers started doubling the lines so the image seemed to increase in resolution as it loaded. I think MSIE started the latter behavior, and that's what modern browsers do. Progressive JPEGs are somewhat similar, but IIRC didn't gain traction until 2000 or so.