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Non-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.
Warning: The Viewport-Width header was removed from the client hints specification in draft-ietf-httpbis-client-hints-07. The proposed replacement is Sec-CH-Viewport-Width (Responsive Image Client Hints).
The HTTP Viewport-Width request header is a device client hint which provides the client's layout viewport width in CSS pixels. The value is rounded up to the smallest following integer (i.e., ceiling value).
The hint can be used with other screen-specific hints to deliver images optimized for a specific screen size, or to omit resources that are not needed for a particular screen width. If the Viewport-Width header appears more than once in a message the last occurrence is used.
A server has to opt-in to receive the Viewport-Width header from the client, by sending the Accept-CH response header. Servers that opt-in will typically also specify it in the Vary header which informs caches that the server may send different responses based on the header value in a request.
<number>
The width of the user's viewport in CSS pixels, rounded up to the nearest integer.
A server must first opt-in to receive the Viewport-Width header by sending the response header Accept-CH containing the directive Viewport-Width.
Accept-CH: Viewport-Width
In subsequent requests, the client might send Viewport-Width header:
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