On most platforms and browsers, `sans-serif` is equivalent to Arial.
However, on Firefox on Ubuntu (and possibly other Linuxes), `sans-serif`
is DejaVu Sans, a much wider font. This creates a larger shift in text
when the custom fonts finally load. Arial is a web-safe font, and
specifying it explicitly gives us more cross-platform consistency, as
well as reducing the layout shift that happens when fonts load.