As of January 9, 2018, Google Fonts had 848 font families, 114 of which are serif faces. Of those, the following 22 can handle Hanyu Pinyin with tone marks.
- Alegreya SC
- Cormorant (Caveat: In the Cormorant fonts, the marks for tones 2-4 are nearly vertical, which may not provide sufficient distinction between them for many readers.)
- Cormorant Garamond
- Cormorant Infant
- Cormorant SC
- Cormorant Unicase
- Cormorant Upright (Caveat: The third-tone mark in ǚ is inverted.)
- David Libre
- EB Garamond
- Gentium Basic
- Gentium Book Basic
- Noticia Text
- Noto Serif
- Pridi (Caveat: The mark for second tone and the apostrophe look very similar.)
While they have all the tone marks needed for pinyin, which is laudable feat on its own, not all of them can really be called pinyin-friendly. Look at the “fà” sequence in “xīfàn” in Alegreya and EB Garamond. It’s better than tofu, but they aren’t prepared to handle sequences that appear in pinyin at all.
Good observation. I wonder, though, if that sort of thing that might be considered a feature rather than a bug. Certainly English has several f-related ligatures (e.g., ff, fl, fi, ffi, and ffl); the presence of those in a font face shows careful attention, not sloppiness.
I’d be interested in learning more about ligature use with languages with scripts with both accent marks and traditions of fine typography (such as French and Czech).