The SVG renderer embeds fonts directly into the SVG as base64 data. This is to give deterministic outputs and load without a network call.
To include your own font, e.g. Helvetica, you must include the Truetype glyphs:
./ttf/Helvetica-Bold.ttf./ttf/Helvetica-Italic.ttf./ttf/Helvetica-Regular.ttf
You must also include an encoded version of these of mimetype application/font-woff:
./ttf/Helvetica-Bold.txt./ttf/Helvetica-Italic.txt./ttf/Helvetica-Regular.txt
If you include a font to contribute, it must have an open license.