In OSX the user can enter accented characters by pressing and holding a key until a popup with different options appear, and special charaters can be typed by combining the key with Option / Option+Shift (for details see [1]). These input methods don't work in LibreOffice Online: for accented characters the popup does appear, but the choice doesn't impact the typed character, and the combinatinos to type accented characters don't work at all. Tested with Safari 10.1.2. [1] https://forlang.wsu.edu/help-pages/help-pages-keyboards-os-x/
On a sidenote, the accented character input doesn't work in LibreOffice, either (see bug 42437). It would probably have to be implemented differently here, though, since it's the browser handling the popup and character choice.
This issue also impacts regional characters in OSX, for some keyboards (PL for instance) R-Alt is used for inputting regional chars. This is not working correctly too. Tested with OSX 10.13.1 on Safari and Chrome. Working normally on Windows Chrome.
Works in formula bar (browser text field), but not in native LO editors, ie. cells or text input in Writer.