When I am trying to open some HTML page, for example Plesk web statistics https://www.DOMAIN.com/plesk-stats, the browser offers me to save the page instead of showing content.
Such problem appears if the type for HTML pages is specified incorrectly.
Make sure that '.html' and '.htm' extensions are not assigned to any type except for 'text/html'. All lines with such types definitions can be found in Apache configuration files with the command line:
# grep htm /etc/httpd/conf.d/* /etc/httpd/conf/httpd.conf | grep AddType
Note, the paths to Apache configuration files can differs depending on OS you use. For more information about apache web server related files location please check http://kb.sp.parallels.com/en/5440