  The median webpage has increased in size by more than 80% in the last 4
years. This extra complexity allows for a rich browsing experience, but it
hurts the majority of mobile users which still pay for their traffic. This has
motivated several data-saving solutions, which aim at reducing the complexity
of webpages by transforming their content. Despite each method being unique,
they either reduce user privacy by further centralizing web traffic through
data-saving middleboxes or introduce web compatibility (Webcompat) issues by
removing content that breaks pages in unpredictable ways. In this paper, we
argue that data-saving is still possible without impacting either users privacy
or Webcompat. Our main observation is that Web images make up a large portion
of Web traffic and have negligible impact on Webcompat. To this end we make two
main contributions. First, we quantify the potential savings that image
manipulation, such as dimension resizing, quality compression, and transcoding,
enables at large scale: 300 landing and 880 internal pages. Next, we design and
build Browselite, an entirely client-side tool that achieves such data savings
through opportunistically instrumenting existing server-side tooling to perform
image compression, while simultaneously reducing the total amount of image data
fetched. The effect of Browselite on the user experience is quantified using
standard page load metrics and a real user study of over 200 users across 50
optimized web pages. Browselite allows for similar savings to middlebox
approaches, while offering additional security, privacy, and Webcompat
guarantees.
