File Extension SSI File Format HTML with server side includes SSI stands for HTML with server side includes. This was last updated in March 2012 By Margaret Rouse Browse File Extensions Alphabetically: A B C D E F G H I J K L M N O P Q R S T U V W X Y Z # Ask a Question. Find an Answer. Ask your peers a question about this file extension. Ask a Question Can the XML Element be assimilated without losing orig document? HTML Recommendations on learning CGI for RPG-backend website?