Currently, the following sequence of actions is allowed and produces strange results:
SourceBuffer.appendMode = sequence;
Append [0,10)
Abort
Append [100,110) // or Append [0,10) again

Without the Abort ( == reset parser state), the expected result would be:
[0,20) (assuming each append began with a random access point such that no frames were dropped).

With the Abort and the current Reset Parser State algorithm that is appendMode-agnostic, the result would be:
[0,10 with a large not-real-gap until 100,110)
// or in the alternate case:
[0,10) from the second Append [0,10).

I believe the correct way to fix this problem is to include the following step in the Reset Parser State algorithm:
'If mode equals "sequence": Set group start timestamp equal to the group end timestamp.'

I will prepare a PR shortly for review.