How to fail to change fonts in librsvg2

On Debian GNU/Linux I am attempting to use librsvg2 to render a few small images (because Mozilla-based browsers cannot display SVG), and the wrong font is being used. How do I find out how to correct this?

Here is a sample file. First, the SVG version (source code: tag.svg):

The font-family attribute specifies 'Helvetica, Arial, sans-serif'. In Adobe's SVG Viewer 3.0 it looks like this:


This uses the font Helvetica; you can tell by looking for the flat top on the t, the curly tail on the a, and the spur (beard) on the G. Batik works fine, but chooses to use Arial rather than Helvetica:


You can tell it's Arial from the slanted t, the less-curled tail on the a, and the beardless capital G. Finally, here's the librsvg2 version:


I don't know what font this is, but it is neither Arial nor Helvetica. More significantly, it has different font metrics. (I think it is Bitstream Vera Sans, the default sanserif font.)

In this particular instance, I want Helvetica (or Arial), because that is (more or less) the font used in the Flickr logo. Also, as discussed yesterday, the difference in the metrics of the font means the word falls off the edge of the picture. Now, neither Helvetica nor Arial are free software, although for a while Microsoft were allowing people to download Arial gratis, but there are other metrically compatible sanserif fonts. I seem to remember using URW Gothic in TeX back in the early 1990s, and Debian has a fc-list command whose output includes references to this font; does that mean it is installed in a way that rsvg can use it? If so, couldn't it be aliased to Helvetica for the sake of simplifying cross-platform SVG?

The librsvg2 web page has nothing to offer on configuring the way it finds fonts. This library is based on Ralph Levien's library Libart; perhaps there will be some clues there? There are some references to fonts -- unfortunately the first one Google found for me was about how the SVG specification makes implementing text needlessly complicated! Oh, well.