I have multiple sites that share a lot of the same files. Sorta like:

CoffeeBeansForSale.com

CellPhonesForSale.com

ComputersForSale.com

They will end up using a lot of the same code / files so it would make sense for both to point to the same info on the server so if I, for example add a new font or update an order form, it wouldn't require updating it on 50 sites. What would be the best way to set this up on the site?

I know in PHP I can create a 'common_site_files' directory and just do an include('/home/mysites/common_site_files/fonts.php'); in PHP. But with HTML it won't really work.

So what would be the best way to set up the sites / directories so I can save myself hours of work in the future?