- The default is 16px.
- If you create an HTML file with any text in it, open it in Chrome, you can check the computed styles.
- I would be interested to see someone test this on mobile versions.

- Edit:- 
As of 2017-03-20, the stable versions of Chrome, Firefox, Edge, and IE on the desktop as well as Chrome, Firefox, and Opera on mobile all default to the de facto standard of 16px for font-size.