The algorithm steps for creating a PresentationConnection set the url attribute after construction. It seems like we should make the attribute optional (like id) since it is not set in the constructor.

Three algorithms that create PresentationConnection objects and set attributes after construction:

• https://w3c.github.io/presentation-api/#starting-a-presentation
• https://w3c.github.io/presentation-api/#reconnecting-to-a-presentation
• https://w3c.github.io/presentation-api/#monitoring-incoming-presentation-connections

Alternatively, we could define a constructor that takes the url and id and modify the algorithms to use that; which seems better, since the API is not intended is to expose PresentationConnections without these properties set.

Reviewing the spec, in all cases where the user agent instantiates a PresentationConnection, it sets both the URL and the ID before returning it to the caller. So it makes more sense to make both PresentationConnection.id and PresentationConnection.url mandatory.