For a long time (years) there was a bug in Firefox that'd prevent a Windows machine from going to sleep if webgl content was loaded in any FF tab.

So anyway that killed one of my laptop's batteries. So much for supporting Internet freedoms...

Windows comes with a utility that'll tell you what process denied a sleep request, super useful.

I've actually ran into MacBooks not sleeping a few times, but it is much rarer.

It is unfortunate because back on the mid 2000s windows had the best functioning sleep code, but then they tried to catch up with iPad's # instant on and chasing perfection led to the current mess.

Macs these days wake up regularly as I understand it. My MacBook's battery discharges decently fast even when the lid is shut