Why does the selection of Flex items go wrong?

Question

css

Hello.
Can you please help me understand why when selecting Flex items from left to right, three left items are selected at once, and not sequentially, as expected (unlike, for example, "inline-block")?
Thanks

JSFiddle Code

Answer 1

It turned out that the problem was in the browser.
Windows 10.

• Chrome - WRONG (Blink)
  
• Opera - Wrong (Blink)
  
• Firefox - works correctly (Gecko)
  
• Edge - works correctly (Edge)
  
• IE11 - works correctly 0_o (Trident)
  

Answer 2

Wrong by what standards? Maybe this is just the expected behavior from the point of view of the browser?