SSI doesn't work. Why?

To assure correct operation of Server Side Includes (SSI) it shall be necessary (by using .htaccess file) to set processing of files with html extension at the server side (by default, SSI operates only in files with shtml extension).

It can be done using .htaccess file which the following lines shall be added into:
    AddType text/html .html
    AddHandler server-parsed .html