I realize this question was answered years ago, but I think it was not the best answer. I just learned a very simple way to include html files inside of other html files that is much better.
The trick is to name the your main .html file as a .shtml file. It is still an html file, but now the server will look inside for other files that you can include this way.
<!--#include file="Preamble.txt" -->
Your Preamble.txt can be another .html file that you want to be included in your main file.
This is called SSI (Server Side Includes). I guess it makes sense that the server needs to look inside the .html file it is sending to a client. I always assumed that it was always automatically done that way and that you didn't need to specify it. Turns out only certain file types are checked for included files by the server.
BTW, I noted the following cryptic answer, but only understood it after I learned about the SSI from other sources. Unfortunately, I got so much bad information in the past, I never realized there was an easy way to make web pages modular. I was doing it wrong for over 15 years.
"Server Side Includes" perhaps? – David Feb 28, 2019 at 11:40