I can't reproduce this at any window size on Firefox, Chrome or Edge on Windows 10. I can hardly fault a typeface designer for not being an expert web developer who can build a consistent user experience on every conceivable browser configuration.
NATIONA
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Now the "Download" at the top, he's got some explaining to do...
(Agree with your point about expert web dev. Someone who designs fonts =/ someone who designs websites. Totally different skills. Some people might do both, but assuming that would be rather bold)
Microsoft got this one right when they released free Windows VMs for various combinations of Windows and IE versions.
Now, yes, it's fairly likely OP is already using macOS and they just didn't bother testing Safari, but if you want to be snarky about browser testing, be snarky to Apple first. They're intentionally making it difficult to test in Safari.