As you probably know, images — in particular JPEGs — make up the
vast majority of a web page’s overall size. The other elements —
text, stylesheets, scripts — usually account for just a few
percent of the total page size. When you load a modern news
website like ExtremeTech, it’s not unusual for a single page to
consist of a few megabytes of data, most of which are images. If
the file size of images could be reduced by just a few percent,
huge speed gains and bandwidth savings could be realized — for
home and office surfers, but more importantly for people on
woefully constrained and metered mobile data connections.
Mozilla’s latest effort, mozjpeg, aims to do that just, by
reducing the size of JPEGs by 10% or more.  The JPEG (Joint
Photographic Experts Group) file format has been around since
1992. It wasn’t originally designed for the web (the World Wide
Web as we know it didn’t exist until 1993, and didn’t become
truly popular until the late ’90s), but it quickly became the de
facto standard for web images