I only looked at the console warnings, not interactions.
Instead of looking at the screen and making determinations with your eyes, look to the source. You can inspect the page on different browsers and operating systems to see which font is loaded, what the size and weight is. When you use devtools you can also disable the browser cache, insuring that you are looking at the code served by the web server.
I just looked at the navbar on OS X in Safari, Firefox, and Chrome. All 14px, Montserrat, normal weight.
Might be different on windows. You can go check. From there you can isolate your issue and adjust your CSS as you see fit. Base styles and fonts can be different on each device, OS, and browser version, and make.