To include HTML (or other extension) files into another HTML file,
<!--#include VIRTUAL="/filetoinclude.html" --> is the line to use.
VIRTUAL is used when file path is not local, such as when "/" is used. Otherwise, it can be replaced by FILE="filetoinclude.html".
For this to work, SSI on web server must be enabled. Most servers such as Apache CPanel default this to shtml. So, to have the Apache CPanel use html extension as the host file as well, add handler: server-parsed and extension: .html
Included files can have any extension, supposedly.
If the host file is php, then maybe it's better to use <?php include("../leftmenu.html"); ?>
The path "../" or full path "http://...." seems necessary, because merely "/leftmenu.html" would not work.