Unity 5.1.1f1 - WebGL : Lock cursor in the screen not working

Hello everyone. I'm working on a game project on Unity 5.1.1f1 and I had to build it on the WebGL platform. In my game management script, I lock the cursor in the screen with the code below. It works in the editor but not after the build and the integration on the website. So when I want to turn the camera on a side, it is sometimes blocked.

It seems to be a recurrent problem on this forum, so I wonder if it is a Unity general problem (because of the new version) or this comes from my code ?

Here's my code.

 private bool lockCursor;    
 
 void Start() {
      lockCursor = true;
 }    
 
 void OnGUI () {
      if (Input.GetKeyDown (KeyCode.Escape))
           lockCursor = false;
 
      if (lockCursor) {
           Cursor.lockState = CursorLockMode.Locked;
           Cursor.visible = false;
      } 
      else {
           Cursor.lockState = CursorLockMode.None;
           Cursor.visible = true;
      }
 }

Thank you very much.