Here is a sample page that prints the key codes:

https://output.jsbin.com/cafabo

Testing Chrome on Linux:
Pressing alt+enter doesn't send any keypress event. The same works in FF.

Testing Chrome on Mac:
Pressing Command+enter doesn't send any keypress event. The same works in FF.

Testing Chrome on Win:
Pressing command/alt+enter doesn't send any keypress event. The alt+enter works in FF but not command + enter.