Interpolates the HTML source with the context, then returns that HTML
        and the text extracted from that html.

def evaluate(self, context):
        """
        Interpolates the HTML source with the context, then returns that HTML
        and the text extracted from that html.
        """
        html = self._source.format(**context)
        parts = {"text/html": html, "text/plain": textFromHTML(html)}
        return {}, parts