Dead keys do not work properly in any browser on any platform. In many cases we get the raw key (see bug 4560) but not the symbol. Some browsers/platforms do local compositing, but some do not. It's a general mess. Upstream report: https://github.com/novnc/noVNC/issues/350
In some cases, the user is not after the dead key itself, but rather the non-dead equivalent, or the composited key. That might be easier to solve than proper support for dead keys. Note that this will likely mean that the server will fail to fake the expected physical key codes. But that might still be a better compromise than the current state where nothing happens for the dead key, and the following key just gives you the non-composited symbol.
