Viewport-Width header

WARNING: The Viewport-Width header was standardized as Sec-CH-Viewport-Width and the new name is now preferred.

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.

Syntax

Viewport-Width: <number>

Directives

• <number>
  • : The width of the user's viewport in CSS pixels, rounded up to the nearest integer.

Examples

Using Viewport-Width

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:

Viewport-Width: 320

Browser compatibility

See also

• Improving user privacy and developer experience with User-Agent Client Hints (developer.chrome.com)
• Device and responsive image client hints
  • Sec-CH-Viewport-Width
  • Sec-CH-Viewport-Height
  • Sec-CH-Device-Memory
  • Sec-CH-DPR
  • Sec-CH-Width
  • DPR
  • Content-DPR
  • Device-Memory
  • Width
• Accept-CH
• HTTP Caching: Vary and Vary header