I’m using three of them in the skrollr source
return window.pageYOffset || document.documentElement.scrollTop || document.body.scrollTop || 0;
https://github.com/Prinzhorn/skrollr/blob/b98d40820b9864be275e81af382045d72cc5a08a/src/skrollr.js#L627
a) So far it’s working across all browsers (nobody complaint in the past year).
b) Since it will use the first one that is defined, I guess it’s pretty future proof and stable.
If you’re fancy you could do this as well
Math.max(window.pageYOffset, document.documentElement.scrollTop, document.body.scrollTop)