This seems to have problems with dead keys. Typing ' and then d on US International results in 'd, but doesn't count as such and instead is an error.
Firefox 81, Windows: Dead keys and the corresponding combination of letters where no single letter is defined by the keyboard layout don't even appear. I.e. I type ', and then s, which normally causes the digraph 's to appear, but here neither the ' nor the s afterwards register.
Chrome 85, Windows: The key sequence registers as just one letter, but apparently not as the ', which thus is marked as an error. This was also what I've seen in Edge (Chromium-based, probably similar version number).
Internet Explorer 11: Same as with Chrome. However, with the added bonus that typing a space seems to be impossible. No idea whether that browser is even on the compatibility list for that site, but it's all I have lying around here right now.