With 2.18, I see this instead:
<text transform="translate(30.7267, 18.7229)"
font-family="Century Schoolbook L"
font-weight="bold"
font-size="6.9848"
text-anchor="start"
fill="currentColor">
<tspan>
aaaa</tspan>
</text>
IMHO, this is as good (or as bad) as a default as 'serif'. If you
want SVG images to be similar to, say, PDF output, you have
* to exactly specify which font you want to use for text, and
* to convert glyphs to paths, for example, by postprocessing
LilyPond's output SVG file with inkscape's `-T` option.
Werner