Vary for Images: Serve the Correct Images to the Correct Browsers

blog.cloudflare.com
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We're excited to announce support for Vary, an HTTP header that ensures different content types can be served to user-agents with differing capabilities.
Today, we're excited to announce support for Vary, an HTTP header that ensures different content types can be served to user-agents with differing capabilities.

At Cloudflare, we're obsessed with performance. Our job is to ensure that content gets from our network to visitors quickly, and also that the correct content is served. Serving incompatible or unoptimized content burdens website visitors with a poor experience while needlessly stressing a website's infrastructure. Lots of traffic served from our edge consists of image files, and for these requests and responses, serving optimized image formats often results in significant performance gains. However, as browser technology has advanced, so too has the complexity required to serve optimized image content to browsers all with differing capabilities — not all browsers support all image formats! Providing features to ensure that the correct images are served to the correct requesting browser, device, or screen is important!

Serving images on the modern web

In the web's early days, if you wanted to serve a full color image, JPEGs reigned supreme and were universally supported. Since then, the state of the art in image encoding has advanced by leaps and bounds, and there are now increasingly more advanced and efficient codecs like WebP and AVIF that promise reduced file sizes and improved quality.

This sort of innovation is exciting, and delivers real improvements to user experience. However, it makes the job of web servers and edge networks more complicated. As an example, until very recently, WebP image files were not universally supported by commonly used browsers. A specific browser not supporting an image file becomes a problem when "intermediate caches", like Cloudflare, are involved in delivering content.

Let's say, for example, that a website wants to provide the best experience to whatever browser requests the site. A desktop browser sends a request to the website and the origin server responds with…
Alex Krivit
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