I have a website which consists of 5 different pages.
To maintain the design of all the pages, I copied and pasted the code from the main page to all the other HTML
Alright so my approach is a bit different as I use PHP but hopefully I am still able to help you with this. I am working on something similar where I have "index" page that includes a nav bar at the top and empty space below it. After I click on something the content of another php file loads into white space, and said another php file is wrapped into a div I can edit with css. To do this I've used this php command:
This should be it. I don't know php very well and one of my collegues suggested to use this but it's relatively small amount of code and it works well.