Source on network vs. Media Source Extensions

[JohnRiv]

Migrated from https://github.com/w3c/webmediaporting/issues/30:

For the issues in this repo, are any of those answers different if the source of the media element is the URL of a media resource on the network rather than MSE?

[chcunningham]

For Chrome I'm aware of a few differences

• For URL, when paused/ended, we elide seeks to the paused/ended time. For MediaSource, we don't elide these seeks because users may use MSE to change the data at the current time.
• For URL, when preload="metadata", we attempt to defer allocating decoder resources until playback starts. For MSE, we don't do this. I'm guessing because loading is now the apps responsibility and we assume they want to use the player immediately. @sandersdan to confirm.

FYI: My method for finding these is just looking at our media player code for mentions of chunkdemuxer (the MSE demxuer). The "player" abstraction sets up the pipeline, which triggers decoder selection. The lower layers of our pipeline (those below the "player", including decoding, rendering, etc), are totally agnostic to MSE vs URL.

[sandersdan]

I'm not certain of the reasoning to avoid deferring MSE. It could be that it's much harder to fake the progress events, and was therefore high risk for launching the intervention. Without progress events, I expect many MSE apps would wait forever, never choosing to enter a playing state.

For non-MSE playback, the rule is that we must reach HAVE_CURRENT_DATA. If we don't get that far then HTMLMediaElement will not enter a playing state even if the app calls play(), and therefore the player never learns that it should disable the intervention. There is not much point fixing that internal limitation because the app could be waiting for the same event too.

[chcunningham]

It could be that it's much harder to fake the progress events, and was therefore high risk for launching the intervention.

We don't fire 'stalled' and 'progress' events for MSE. We used to, but it seemed not useful and not to spec. https://groups.google.com/a/chromium.org/g/blink-dev/c/x54XtrTyOP8/m/4-5QZlZzDAAJ