WidthWidth is a Client Hint request header that reports the physical pixel width at which the requested image resource will actually be rendered. The server can then choose an image of exactly the needed size.
The value is the resource's intended physical width in device pixels (e.g. `Width: 640`), which the browser computes from layout and DPR.
Whereas Viewport-Width reports the whole screen width, Width reports the actual pixel width an individual image will occupy within the page. The browser computes the final physical width by multiplying the layout width expressed in the `sizes` attribute by the device pixel ratio (DPR) and attaches it as the Width hint on that image's request. The server uses it to select exactly the right resolution, avoiding over- or under-sending.
Like other Client Hints it is opt-in: the server must request `Accept-CH: Width` (and commonly `DPR`, `Viewport-Width`) before the browser attaches the value to image requests. When responses vary by the hint, send `Vary: Width` so caches don't mix resolutions.
This hint is especially powerful with DPR: the server directly receives the physical width that already reflects DPR against the layout width (CSS pixels), delivering crisp images on high-density displays without wasting bandwidth. Since the unprefixed legacy name is on a deprecation path, prefer `Sec-CH-Width` for new implementations.
Width: <integer>e.g. Width: 640