Here’s a nice demo page for Source Serif 4 which illustrates its versatility.
Source Serif is an open-source typeface for setting text in many sizes, weights, and languages. The design of Source Serif represents a contemporary interpretation of the transitional typefaces of Pierre-Simon Fournier. Additionally, Source Serif has been conceived as a friendly companion to Paul D. Hunt’s Source Sans.
Back when I first started using Source Sans Pro for text for my personal website, I tried pairing it with Source Serif for headings. It didn’t quite work for me then but the typeface seems to have undergone some changes in the interim and also now comes with a variable font option. I might give it another spin.