>>> I think, perhaps, introducing a new visible bell (across all 
>>> significant system), that's considerably less intrusive than the ones 
>>> we have today on GNU/Linux and Macos, might be an option.  And then 
>>> defaulting to using that.
>>>
>>> Gregory suggested blinking the cursor in a different colour -- 
>>> perhaps?
>>
>> I attach a POC of what I have in mind.
>
> If this is being proposed as the default setting, then we should have a 
> different default for TTY frames, because AFAIR cursor color cannot be 
> changed there, right?
>

Indeed.

>>   (sit-for 0.5)
>
> I think half a second is too long, a bell should be much shorter, like 
> 0.1 sec or something.
>

Hmmm... 0.5 seconds is the duration of typical audible bells.  And with a 
sit-for, it's at most 0.5 seconds, when the user presses a key the bell 
signal disappears.  That being said, I agree with you that 0.5 seconds is 
perhaps a bit too long, so I changed it to 0.25 seconds.

I attach the updated POC, which works on non-graphical displays.