In Leap 42.1, when I don't choose any specific font faces in either my
browser's preferences or a page's CSS stylesheet, what I get as system-default
serif font is RobotoSlab, see
https://bugzilla.mozilla.org/show_bug.cgi?id=1228221#c19

Now this is a slab-serif font, which is admittedly within the meaning of
"serif" as defined by the CSS definitions at W3C, but it is certainly not what
I think of when I write "font-family: serif" in a style sheet. FreeSerif,
another font which is also installed, looks much more like what I think of as
an "ideal serif font", and what is more, it positions its combining accents
correctly over the letters they concern (see again the above-mentioned Mozilla
bug).

I move that the system default serif font in future Leap distributions be
FreeSerif rather than RobotoSlab.