UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.0.2987.98 Safari/537.36

Steps to reproduce the problem:
1. Any function that handle DOM events, example:
document.body.onclick=function(e) { console.log(event); }
2. click event is triggered
3. the console log a MouseEvent Object

What is the expected behavior?
Should give an error
(Uncaught ReferenceError: event is not defined) 

What went wrong?
This proprietary is non-standard, and Chrome supports it because of compatibility to old IE, but at least the developer should be warn of this non-standard usage, or given an error to be more compliant with standards.

Did this work before? N/A 

Chrome version: 57.0.2987.98  Channel: stable
OS Version: 10.0
Flash Version: 

Developers could use the proprietary without intention to use it, example using 'e' in the function declaration but 'event' inside the function, example:
document.body.onclick=function(e) { console.log(event); }