[whatwg] <video> resource selection algorithm and NETWORK_NO_SOURCE

Silvia Pfeiffer silviapfeiffer1 at gmail.com
Fri Jul 23 20:51:12 PDT 2010

There is definitely a spec bug, because different locations of the spec say
diverging things. While the first step in the resource selection algorithm
states to go into NETWORK_NO_SOURCE state, the description of
NETWORK_NO_SOURCE in the spec actually implies a state of failure. Since the
first step in the resource selection algorithm really is a state where we
know nothing about the resource yet, it matches more with the description of

NETWORK_EMPTY (numeric value 0)
    The element has not yet been initialized. All attributes are in their
initial states.

NETWORK_NO_SOURCE (numeric value 3)
    The element's resource selection algorithm is active, but it has failed
to find a resource to use.

In any case, it would be nice to get the spec corrected either way and the
implementations aligned. :)


On Sat, Jul 24, 2010 at 12:16 AM, Philip Jägenstedt <philipj at opera.com>wrote:

> Silvia made we aware of discrepancy in how browsers implement the resource
> selection algorithm, see forwarded message. It's my assessment that Opera is
> the only browser following the spec. I've filed this bug with Mozilla:
> https://bugzilla.mozilla.org/show_bug.cgi?id=581355
> I've also reported bugs in Chrome and Safari, but can't see where they
> ended up.
> The reason I'm writing this email is that apparently everyone but myself
> has a different interpretation of the spec, so perhaps this is something we
> need to discuss. Does any other browser ever set the state NETWORK_NO_SOURCE
> at all? I speculated that perhaps other browsers aren't very strict about
> which parts of the algorithm are run synchronously and not, but even
> checking the networkState after a setTimeout it still isn't
> Test case: http://people.opera.com/philipj/2010/07/23/networkState.html
> Please fix implementation or spec :)
> --
> Philip Jägenstedt
> Core Developer
> Opera Software
> ------- Forwarded message -------
> From: "Silvia Pfeiffer" <silviapfeiffer1 at gmail.com>
> To: "Philip Jägenstedt" <philipj at opera.com>
> Cc:
> Subject: Re: bug in Opera video
> Date: Thu, 22 Jul 2010 12:51:15 +0200
> Hi Philip,
> On Thu, Jul 22, 2010 at 8:02 PM, Philip Jägenstedt <philipj at opera.com>
> wrote:
>> On Thu, 22 Jul 2010 01:23:09 +0200, Silvia Pfeiffer
>> <silviapfeiffer1 at gmail.com> wrote:
>>  Hi Philip,
>>> I've just made an experiment with the play() function in Opera and it
>>> seems that Opera doesn't support play() if currentSrc has not been
>>> determined yet (i.e. if the media element doesn't have a loaded
>>> resource). The spec says to deal with this situation and so do all
>>> other browsers, bar Opera. Here is a test:
>>> <!DOCTYPE html>
>>> <html lang="en">
>>>  <body>
>>>   <video controls width="400px">
>>>     <source type="video/mp4">
>>>     <source type="video/webm">
>>>     <source type="video/ogg">
>>>   </video>
>>>   <p>currentSrc on start: <span id="currentSrc_first"></span>.</p>
>>>   <p>currentSrc after loadedmetadata: <span
>>> id="currentSrc_loadedmetadata"></span>.</p>
>>>   <script type="text/javascript">
>>>     video = document.getElementsByTagName("video")[0];
>>>     source = document.getElementsByTagName("source");
>>>     span1 = document.getElementById("currentSrc_first");
>>>     span1.innerHTML = video.currentSrc;
>>>     source[0].src = "HelloWorld.mp4";
>>>     source[1].src = "HelloWorld.webm";
>>>     source[2].src = "HelloWorld.ogv";
>>>     video.play();
>>>     span2 = document.getElementById("currentSrc_loadedmetadata");
>>>     function span2Update(evt) {
>>>         span2.innerHTML = video.currentSrc;
>>>     }
>>>     video.addEventListener("loadedmetadata", span2Update, false);
>>>   </script>
>>>  </body>
>>> </html>
>>> The video.play() function should execute the resource selection and
>>> fetching, but it doesn't. Not sure if this is deliberate and you want
>>> a discussion on the whatwg about it, or just a bug.
>>> Cheers,
>>> Silvia.
>> The spec says "If the media element's networkState attribute has the value
>> NETWORK_EMPTY, invoke the media element's resource selection algorithm."
>> I added alert(video.networkState) just before video.play. In Opera it is 3
>> (NETWORK_NO_SOURCE) while in Chrome it is 0 (NETWORK_EMPTY).
> Ah, yes, this is actually another thing I wanted to alert you to. All
> other browsers set the @networkState to NETWORK_EMPTY at the start of
> loading a new media resource (Firefox, Safari and Chrome). I was going
> to discuss this with you, since Opera interprets the spec different
> here - which is understandable, since the first step in the resource
> selection algorithm says to:
> 1. Set the networkState to NETWORK_NO_SOURCE.
> OTOH however, the description for the NETWORK_NO_SOURCE state is as
> follows:
> NETWORK_EMPTY (numeric value 0)
>     The element has not yet been initialized. All attributes are in
> their initial states.
> and for NETWORK_NO_SOURCE is:
> NETWORK_NO_SOURCE (numeric value 3)
>     The element's resource selection algorithm is active, but it has
> failed to find a resource to use.
> Thus, I believe this may be a spec bug and really the first step in
> the resource selection algorithm should say
> 1. Set the networkState to NETWORK_EMPTY.
> which is more consistent with the implementations of the other
> browsers and also consistent with the meaning of NETWORK_EMPTY and
>  We are
>> following the spec here, as the resource selection algorithm should be run
>> the first time when the video element is inserted into the DOM by the
>> parser, then again after each source element is inserted. For this markup,
>> it will stay as NETWORK_NO_SOURCE in all cases, which is basically a
>> waiting
>> state.
>> There are a few ways we could go about solving this:
>> 1. Make modifying the src attribute of source elements that are children
>> to
>> a video element that has state EMPTY or NO_SOURCE run the resource
>> selection
>> algorithm. I don't like this idea myself, it's unnecessarily complicated.
>> 2. Let the other browsers follow the spec and add a call to video.load()
>> before video.play().
>> Please do bring this up with the WHATWG. If you like you can forward this
>> mail verbatim. To anyone testing this, note that HelloWorld.mp4 has to
>> actually exist (or changed to something that does) for the above test to
>> work.
> Actually, in cases where it doesn't exist, all other browsers can deal
> with it, but Opera hangs. I have actually posted that to Opera bug
> reports if you want to follow up on this, too.
> I'm leaving it to you to bring it up at WHATWG, since it seems that
> the Opera implementation is the odd one out. :-)
> Cheers,
> Silvia.
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