For this you need to create a file named .htaccess and put that file in your /htdocs/www/ directory.
In the .htaccess file you need to add ;
AddHandler server-parsed .html
Now all files in any directory under /htdocs/www/ with filenames ending in .html will be parsed as Server Side Include files, just like they would be if they had an .shtml extension instead.
If you only want this effect in a certain subdirectory tree, just put the .htaccess file in that subdirectory instead of putting it in /htdocs/www.
To turn this effect off in some subdirectory where it has been turned on by an .htaccess file in a directory above it, make a new .htaccess file in the directory where you want to turn it off that has this 1 line:
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