Would you still build him with 
but throw in a 
or 
third? And if you were to do Tri third, how would he benefit from it? Also would sticking with 5 crit red's still be viable at that point since you'll have 80% with IE+Shiv and 100% if you build Tri/Youmuu's third (5% from runes)?
No no child, 
is the new way to go. People just don't seem to realize how powerful this item is, just because Shiv is fancier with a cute little stacking active.
phreak did something like 6 crit red and one crit yellow in his rune page guide
I'm just going to deal with the 90% Crit. It isn't that big of a deal and going so far out of your way just to reach 100% is not ideal in my opinion.
Yasuo really enjoys life steal so when u replace that with tri force or youmuu's it may be impossible to get life steal assuming that u get boots and last whisper.
+ = 100% Crit. No runes required.
Hell, you could be lazy and just build 2 ! 
you can fill it in with runes if you really care that much. 90% is probably enough though, it's not really that big a deal for him.
Build ghost blade
I have faith in the power of RNG that the 90% crit rate will be on my side
I find statikk shiv's lighting is super effective for clearing waves. The damage will always crit because of Yasuo's passive. Therefore, you have 2 solutions:
+ + 10% of crit chance runes = 100%
OR
+ + = 100%.
Even I'm not sure which. But since I don't have the IP for runes right now, I'd give triforce a try.
