SD and HD resolutions in a single HLS playlist?

"The framerate for all streams should match. The resolutions for HD streams should match and the resolution for all SD streams should match."

The resolutions for all HD streams should match - meaning for HD mode, if you have 1280x720, they should all be 1280x720.
And
The resolutions for all SD streams should match - meaning for SD mode, if you have 720x480, they should all be 720x480.

The reasoning behind it is - it's quite annoying to the end user if they start a 1280x720 stream and it rebuffers and becomes a 360x240 stream on the tv.

Typically for HLS there will be one playlist with all the variant streams in it. These streams, while some may be HD and other SD should all have the same aspect ratio, so the SDTV should match the HDTV, otherwise, you'll see the image jump in and out when the bandwidth fluctuates.

All segments should be aligned - in other words each 10 second chunk in any bitrate should show the same 10 seconds of video, otherwise there will be nasty macroblocking artifacts when the bitrate switches.

- Joel

Thanks. I'd hastily copied the SD and HD resolutions from the Encoding Guide without checking the aspect ratio so inadvertently turned this a slightly different question to that I meant. Aspect ratios, keyframe and frame rates and timings of streams in a single playlist naturally need to match, as with any player.

Given all streams should be in a single variant playlist (which I'd hoped was the case), why should there be only one HD resolution? Wouldn't including 1280x720 and 1920x1080 be useful to provide a native resolution for the two common HD screen sizes? For that matter does Roku factor in resolution when adaptive switching with HLS, or is only bandwidth that matters?