It is difficult to decide whether to use pixels, ems, or rems for typography. It's a matter of style. Some web designers/developers still use pixels as their unit to declare font sizes. It's just a lot easier to wrap our heads around the sizes.
The issues with setting font sizes in pixels were basically on legacy IEs, where, if the user wanted to zoom in on the page for whatever reason, the text would stay fixed at the pixel size it was given.
Now, that's a thing of the past as far as modern browsers are concerned. When you zoom in any modern browser, if it's zoomed in enough, it will trigger the media queries, hence showing the mobile version of the site.
Another problem with pixel-based font sizing is that it's hard to scale and maintain. What this basically means is that we'd have to declare the font sizes of many more elements in every media query, over and over and over.
On the other hand, we have relative units, ems and rems, which are pretty much the...