Can you check how much room remains in a WebSocket’s send buffer?

Regarding a WebSocket’s send() method, the spec has this to say:

…if the data cannot be sent, e.g. because it would need to be buffered but the buffer is full, the user agent must flag the WebSocket as full and then close the WebSocket connection…

You can check how much data has been buffered without being sent to the network by reading a WebSocket’s bufferedAmount property, but how can you tell how many bytes remain in the buffer before it becomes full?