That's exactly what we've done here on CodePen!
If we detect a major syntax error (nothing just stylisticly off, like a missing semi-colon) we'll highlight the line and show you an error icon. Just like we do with preprocessor syntax errors.
Click the icon for information on exactly what we think is wrong on that line:
This prevents you from having to open the console to look for errors when things aren't happening as you expect. But you can still use the console of course! We just hope this helps the editing experience and draws attention to errors before the normal hair-pulling confusion sets in.
Enjoy! Let us know if you have any issues with it.