Refresh: main nav doesn't have space between items in some older browsers

[craigcook]

Description

While testing fallback states for variable fonts in older browsers (#15397) I noticed some weren't adding space between menu items in the main nav. I didn't dig into debugging it but my first guess would be maybe we're using some newer flex property that wasn't yet supported. Just wanted to note it and grab a few screenshots so we can circle back to it at some point.

Chrome 59 on Windows 8:

Safari 13.1 on macOS Catalina: