Security HTTP headers, caching, compression, obsolete and useless headers, header consistency, 304 server responses are checked.
Correct HTTP headers increase security and trust in the site, including from search engines, can affect the site’s position in Yandex and Google, save server resources, reduce server load, thereby increasing the server response speed, which again affects the ranking of the site in the search, save money on payment powerful hosting, which may not be required for the site when configured correctly.
Check not only web-page headers, but also headers of static content, images, css and js files. Check separately If-Modified-Since and If-None-Match for correct your web server reply 304 Not Modified.
For detail information see «tuning HTTP-headers» и «setup HTTP2 for web-server Apache on Linux CentOS 7».
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