Using a default font for text?

Is there a way on a TextFlow object to assign a default font to any text that does not have a font specified?

For instance, if the text elements are not wrapped in a font tag, or its ancestor tags are not wrapped in a font tag, to set that text to be a certain font?

<p>This is an unstyled sentence.</p> <!-- I want to style this text with default font -->

<p><font face="Arial" size="12">This is a styled sentence.</p> <!-- but not this one, since it already has a font -->

Any ideas?

Thanks.