can message to the browser window tab.
- Safari Desktop 5.1+
- Safari Extensions JS
The before-load event you are responding to.
The body of the message. Optional.
Use this method to set up an observer for before-load events and call this method from the observer. Then use the return value to respond to the before-load event, permitting or blocking the resource from being loaded.
This message is sent synchronously, unlike all other messages.