If your web server is using suPHP add following line in .httaccess.
AddHandler x-httpd-php .html .htm
OR
AddType application/x-httpd-php .html .htm
If apache is executing php as CGI use following.
AddHandler application/x-httpd-php5 .html .htm
If your server is using PHP5 try following.
AddType application/x-httpd-php5 .html .htm
OR
AddHandler application/x-httpd-php5 .html .htm
If nothing works, try one of the following.
RemoveHandler .html .htm AddType application/x-httpd-php .php .htm .html
<FilesMatch "\.html$">
ForceType application/x-httpd-php
</FilesMatch>
AddHandler fcgid-script .html FCGIWrapper /usr/local/cpanel/cgi-sys/php5 .html
Options +ExecCGI AddType application/x-httpd-php .php .html AddHandler x-httpd-php5 .php .html