[whatwg] When to stop <video> elements from playing

Jonas Sicking jonas at sicking.cc
Fri Oct 26 13:15:23 PDT 2007

A couple of comments:

The spec currently doesn't say to set the autoplaying flag to false when 
an element is removed from the Document. I take it that will mean that 
the element will start playing if it's currently waiting for data? This 
seems undesirable to me for two reasons:

A) I think nodes not in the document should only start playing when more 
explicitly asked to.
B) It creates a race condition where the element _will_not_ play when if 
enough data had been downloaded at the point when the element was 
removed, but _will_ play if it was still waiting for data.

Is there any reason we couldn't state the removing the element from the 
Document calls pause() on the element?

I think that it is still currently possible that a currently playing 
element can get garbage collected. Alternatively, that it is possible to 
create elements that will never stop playing, even if the user navigates 
away. Consider the following scenario:

1) Page A opens a new window containing Page B
2) Page A creates a reference to Page B
3) Page B creates an <audio> element that doesn't live in its Document
    and sticks it in a global variable of its context.
4) Window containing Page B is closed. Page B does not go away since
    Page A is holding a reference to it.
5) Page B calls .play() on the <audio>
6) Page A drops reference to Page B
7) Garbage Collection runs
8) User leaves Page A
9) Garbage Collection runs

The only step here that is slightly suspicious is step 5 since a 
closed-but-alive page is running script. I believe this can happen in a 
number of ways, the simplest being that Page A calls a function on Page 
B. But I suspect there are also events that can fire in Page B even 
after it has closed.

The question is, when does the <audio> element stop playing? If it stops 
playing in step 7 we still have the situation that it can stop running 
at a seemingly random point in time, i.e. when GC runs. Step 6 isn't 
necessarily even needed since both Page A and Page B would get destroyed 
in step 9 anyway.

If we say that the <audio> should not get garbage collected in step 7 
since it is playing audio, at what point should it stop playing? There 
is no connection between the <audio> and Page A so step 6 and 8 won't be 
special in any way to the <audio>.

I believe the best solution here is to say that step 5 should throw an 
exception. I.e. you can't start playing a media element whose 
.ownerDocument is not a displayed document.

Other suggestions welcome.

/ Jonas

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