Reboot does not assign "font-size: 16px;" to html in alpha 6 was fine in alpha 5
Windows 10 and MacOS
Chrome and Safari
I performed a search for all cases of ' html' in bootstrap.css. No assignments to font-size found
I investigated because many headings and anchor tags had suspiciously large font size.
Take a look there: #21524
Then this looks like bug in docs ;)
Closing out per previous comments. You shouldn't have too many problems though, as 16px is normally the browser default. Check that you haven't changed your default text size or enabled zoom.