The file extension SHT marks HTML files with server-side includes (SSI). These are special html files that contain the instructions (script commands) that the web server should execute before the finished page is sent to the client, ie browser.
The instructions usually beginning with '<! -' are ignored by most servers on files ending in htm or html. A typical application is the integration of date and time. Alternative endings are shtml and shtm.
SHT files can be opened like html files in any text editor and web page editor, but they can only be used by SSI capable servers, for example Apache or Microsoft IIS.