640w

640w is a width descriptor commonly used in the HTML srcset attribute to provide the browser with multiple candidate images of varying widths. When an image tag contains something like srcset="small.jpg 320w, medium.jpg 640w, large.jpg 1024w", each image file is labelled with its intrinsic width in CSS pixels using a trailing “w”. The browser uses these descriptors to decide which image to download and display based on the device’s viewport size and pixel density, thereby optimizing performance and visual quality.

The width descriptor is part of the Set-Cookie specification for responsive images. It signals the intrinsic width of the image, not the size it will be displayed at. The browser calculates the effective pixel density that the image will be displayed at and selects the most appropriate candidate that matches the required resolution. For example, on a high‑resolution (Retina) device, the browser may prefer the 1024w variant even if the layout width is 640px, because the doubled pixel density requires a higher‑resolution image to avoid blurriness.

Using 640w as a specific option is common for medium‑sized images on desktop and laptop screens. It often serves as a breakpoint in responsive design frameworks. When combined with the sizes attribute, such as sizes="(max-width: 600px) 100vw, 640px", the browser can further fine‑tune which width descriptor to use. The 640w image will be selected for viewports larger than the breakpoint but not large enough to justify higher‑resolution files.

Performance benefits of the width descriptor include reduced data usage on mobile networks, as the browser does not download unnecessarily large images, and faster initial rendering times. It also mitigates layout shift by providing the browser with size information ahead of time, enabling more accurate space allocation for images while the page loads.

Overall, 640w is a useful tool in modern web development for delivering appropriately sized images across a range of devices, ensuring both efficiency and visual fidelity.