Last-Modified and If-Modified-Since headers are highly recommended but they are not mandatory,
of missing they will be highlighted with yellow in the report.
ContentContent report checks page titles, page headings (H1), HTTP response headers Last-Modified and If-Modified-Since.
Why you need Last-ModifiedHTTP response header Last-Modified contains the date and time at which the origin server believes the resource was last modified. The received Last-Modified value is recorded. The next time a crawler tries to access the page, 'If-Modified-Since' request header is sent by the browser to specify the last modified date of the cached copy of the resource (saved in the browser).
Еhe server will send back the requested resource, with a 200 status, only if it has been last modified after the given date. If the request has not been modified since, the response will be a 304 without any body.
Search engines recommend to send Last-Modified and process If-Modified-Since headers in the correct way.