Client hints - MDN Web Docs Glossary: Definitions of Web-related terms
accept-ch: dpr, width, viewport-width, downlink and / or <meta http-equiv="accept-ch" content="dpr, width, viewport-width, downlink"> when a client receives the accept-ch header, if supported, it appends client hint headers that match the advertised field-values.
... for example, based on accept-ch example above, the client could append dpr, width, viewport-width, and downlink headers to all subsequent requests.
... example varying response: vary: accept, dpr, width, viewport-width, downlink see also client hints headers vary http header ...

HTTP headers - HTTP
viewport-width a number that indicates the layout viewport width in css pixels.
... if viewport-width occurs in a message more than once, the last value overrides all previous occurrences.

Cross-Origin Resource Sharing (CORS) - HTTP
xample, connection, user-agent, or the other headers defined in the fetch spec as a “forbidden header name”), the only headers which are allowed to be manually set are those which the fetch spec defines as a “cors-safelisted request-header”, which are: accept accept-language content-language content-type (but note the additional requirements below) dpr downlink save-data viewport-width width the only allowed values for the content-type header are: application/x-www-form-urlencoded multipart/form-data text/plain no event listeners are registered on any xmlhttprequestupload object used in the request; these are accessed using the xmlhttprequest.upload property.

Content negotiation - HTTP
viewport-width indicates the layout viewport width in css pixels.

Accept-CH-Lifetime - HTTP
syntax accept-ch-lifetime: <age> examples accept-ch: viewport-width, dpr accept-ch-lifetime: 86400 ...

Accept-CH - HTTP
syntax accept-ch: <list of client hints> examples accept-ch: dpr, viewport-width accept-ch: width accept-ch-lifetime: 86400 vary: dpr, viewport-width, width note: remember to vary the response based on the accepted client hints.