It seems to be exclusive to chromium, I'm sure that Firefox and Internet Explorer don't do it this way. They don't start downloading until after I select a location for the download.

Most browser nowadays don't even prompt for a download folder by default. They just start the download as soon as you click on a download link.

I mean if I click a link thinking it will take me to a page where I can read details about a download but get the actual download instead

Well, if you click on a link that says "download something" then it means that you want to download it.

Or if a site is trying to force me to download something without asking me.

And why a serious site would do it?

n either case Opera has already started the download before I can click cancel.

Opera works that way since the old Presto era. And i'm pretty sure that other browsers do the same.