How we improved image download sizes on Medium with just four characters

Adding 'auto' to the sizes attribute on lazy-loaded images lets the browser select the most optimized image variant based on layout, connectivity, and dimensions — without developers writing complex breakpoint rules. WebKit added support for sizes='auto' on March 24, 2026, following Firefox/Gecko. The change is backwards-compatible: browsers that don't support it simply ignore the unknown value. Medium applied this to their home feed and saw roughly 7% smaller image download sizes. WordPress has already updated all lazy-loaded images in their codebase to use it. The practical change is prepending 'auto,' to existing sizes attributes on any lazy-loaded img tag.