My wife also buys a lot of clothes online, and indeed it's a matter of fitting and sending back what doesn't fit. I recently did it too: ordered two pairs of pants, one was fine, the other didn't look remotely like what I ordered, so I sent it back.

In shops, you can try them on. Online, I think it's unavoidable that people try them on at home and send more than half of their clothes purchases back.

Standardisation in the clothing business seems incredibly unlikely. And even when the size is technically correct, it can still be too uncomfortable. You just never know until you try it on.

If you want a truly universal sizing standard that will always guarantee correct sizes, it's going to have to be a lot more detailed than just one or two numbers. It would be a data dump nobody can remember. It might be interesting to have an online service that would remember such detailed sizing info for you and could produce clothes on demand with perfect fit.