hinclude.js
Tired of regenerating HTML pages from templates? Want more from Web caches? HInclude makes one thing very easy; including other bits of HTML into your Web page, using the browser.
HInclude is declarative client-side inclusion for the Web; it allows easy composition of Web pages using the browser -- making your pages more modular, more cacheable, and easier to maintain.
See the demo page for documentation and examples.
If you're interested doing this with Web Components, see Gustaf Nilsson Kotte's h-include.