This is normally done on the server, by whatever tool is building the final html pages. If it’s just a static website (doesn’t take user input, glorified brochure), then one might use a static site generator (eg Jeckyll), each of which have their own mechanism for sharing common snippets. If building a more dynamic website with a database backend, then one would be using other tool (eg. Ruby on Rails), which would also each have their own mechanisms for sharing common snippets.
On it’s face, this is a very odd request. I feel like, in trying to simplify the question, you’ve left out a lot of pertinent details.
My suspicion is that you have a specific problem you’re trying to solve, and, due to lack of experience with web development, you’ve settled on this solution of using JS to copy an html snippet from one document to another, when a proper solution to the actual problem is probably nothing like that. Without knowing what the original problem is and what environment the code would be running in, I’m afraid it’s going to be nearly impossible to offer any suggestions.