If you use dynamic pages like JSP or asp.net, you can have your page template included, and then content added. But what if you have no server-side component and all pages are just HTML/JS? You can of course create a template then copy it for each page, but then if you want to change something you risk having to modify every page, even if you put most styling in CSS properly.
Are there any non-awful ways to do this? I could see that an iframe could be used to load the content into the central page but that sounds nasty. Does HTML provide any way to include a base file and add to it?