There are two ways, both of them require changing settings in the server configuration or an htaccess file:
#1: You do
use the shtml extension but use a 301 (premanent) redirect through htaccess. Look up a search engine of your choice to learn what and how to do this.
#2: You set up a rule (again, in an htaccess file – or by changing the server config if you’re able to) to treat files with the html extension as SHTML files. Look up “enabling SSI” on this page: http://www.buildwebsite4u.com/advanc...ess-file.shtml
(specifically the line in the second box)