With pure HTML, this is not possible; HTML is simply a markup language, and is not extensible.
However, XHTML is extensible (see: Developing DTDs with defined and extended modules).
You could also utilize CSS and/or JavaScript files (which your business partner could reference with <link>
and <script>
elements, respectively), which could modify the appearance (CSS) and behavior (JS) of normal HTML. There are a number of libraries using these techniques already which you could potentially use. For example, Kendo can transform your "normal" HTML elements into something feature-rich.