That can get messy and confusing if the user's locale is different from the language of the web page.

When I write in English, I of course also want to edit dates and numbers using English conventions. But instead, I am forced to use decimal comma and day/month order because those are the default locale for Swedish, which is my default locale. I have never encountered an OS that doesn't work that way. On the web you'll often don't know: it could be anything.