In HLS, chunks do not have to start with keyframes, so to smooth transition from one quality to another, the client must download the duplicate chunks in both the old quality and the new quality and then link them together when there are keyframes In the case of). melt. This is inefficient and significantly increases the probability of re-buffering, especially when the client attempts to switch to lower quality due to a reduction in available bandwidth.
It doesn't appear you have a question here.
It is true that segments do not have to start with a key frame.
However, since early 2014 we have allowed the EXT-X-INDEPENDENT-SEGMENTS tag which signals to the player that all the segments do start with key frames. This allows more efficent processing in that case.