Skritter (and most other online ressources) says it's written with the middle stroke first, others write the left part first.

Which is the correct one? Anyhow, how I find the official version? (Like Oxford Dictionary for British English, Duden for German are setting the standards.)

Sources:

1) "Handwriting" of (chinese) people:
http://www.sayjack.com/chinese/character/乘/

2) Wenlin

Skritter is correct. It should be written with the middle stroke (shu4) first:

http://www.archchinese.com/chinese_english_dictionary.html?find=乘

I notce the stroke order diagram in Pleco shows the left part written first, then the middle. I think it must be an accpetable alternative.

We licensed our current stroke order data from Wenlin, actually, so our agreeing with them on a particular point of stroke order should not be interpreted as confirming it. (we are looking at sourcing some alternate 楷体-friendly stroke order data from a different source for a future update, though)