When I learned CSS over a decade ago, the standard (and only) way to use the "default fonts" (whatever that means) was:
The CSS Fonts Module Level 4 draft introduces new generic font families
For most languages and scripts, existing generics can be mapped to something comparable to that script. That’s useful for the web so that generics like
sans-serifmap to something reasonable independent of the language of the content. But typographic traditions vary widely across the world and many scripts have a variety of common typeface designs that don’t map cleanly into these existing generics. In situations where a similar typeface is used across a wide variety of platforms, it’s useful to extend the set of predefined CSS generic font families to cover these type styles.
If you want to get "the font that feels the least out-of-place on the target platfom", that would be
system-uifont family representing the default UI font on a given platform