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Serif is considered more readable on print and on higher resolution displays (300 DPI or more, e.g. retina display) and these days a big proportion of displays fits into this category. Look at big online publishers like New York Times or medium.com, they even use serif in summaries and in titles.

But what about the original question that was asked... can you change the font in the stories (not should you)? Medium uses a serif font for text and a sans serif for captions. I'd like to duplicate that here.