You might be serving content that depends on the specific browser version a user is using (either because the content itself is relevant to the browser, or because you need to work around behaviour in specific known versions), or you might be serving content that depends on the user's specific OS and OS version (offering a Mac download to Mac users and a Windows download to Windows users). The server responds with the content requested, and includes an Accept-CH header, such as: Accept-CH: Viewport-Width - the server wants to know the width of the client's screen Accept-CH: Width - the server wants to know the desired width of resources being requested (e.g.