In step 8 of https://w3c.github.io/webrtc-pc/#dom-rtcpeerconnection-createdatachannel:

8. If both the maxPacketLifeTime and maxRetransmits attributes are set (not null), throw a SyntaxError.

This error must not be a SyntaxError, which defined as:

19.5.5.4SyntaxError
Indicates that a parsing error has occurred.

The correct error to use is TypeError. Given the same case, applied to descriptor properties:

Object.defineProperty({}, "foo", {
  get() {
    return 1;
  },
  writable: false
});

Will have such results as:

• Firefox

  TypeError: property descriptors must not specify a value or be writable when a getter 
  or setter has been specified
  

• Chrome

  Uncaught TypeError: Invalid property descriptor. Cannot both specify accessors and a 
  value or writable attribute, #<Object>
  

• Safari

  TypeError: Invalid property.  'writable' present on property with getter or setter.
  

I recommend throwing a TypeError with message "Invalid options, maxPacketLifeTime and maxRetransmits cannot both be null" (or something similar)