Currently the spec requires that readyState transitions from "ended" to "open" when a seek occurs. This is a hold over from the earliest versions of the spec and was originally added to allow appending more data after a seek since appending in the "ended" state wasn't allowed. 

Now that you can use append() to transition from "ended" to "open" (see https://www.w3.org/Bugs/Public/show_bug.cgi?id=18708), I don't think this automatic transition is necessary anymore. It also prevents the annoying requirement to call endOfStream() again on every seek if the application is done appending data and wants to stay in the "ended" state.