I’ve found ODF is a very convenient format for generating documents. It has a little bit of a learning curve, but you can edit your templates in Libre Office and fill it using any kind of simple templating solution.

It really is quite neat — if you define all your styles declaratively then the markup you touch is very clean and simple, e.g. something like this:

    <office:document-content xmlns:office="…" xmlns:text="…">
      <office:body>
        <office:text>
          <text:p style-name="foo">Bar</text:p>
        </office:text>
      </office:body>
    </office:document-content>

I’m sure OOXML can be used in a similar manner, but I don’t know that format very well.