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[whatwg] Script-related feedback

From: Steve Souders <whatwg@souders.org>
Date: Wed, 17 Mar 2010 14:44:53 -0700
Message-ID: <4BA14D55.5060600@souders.org>
>  Given that it is possible to do this from script, how common is it for
>  people to do it from script? If it's very common, that would be a good
>  data point encouraging us to do this sooner rather than later.

6 of the top 10 US web sites load scripts after the load event: eBay, 
Facebook, Bing, MSN.com, MySpace, and Yahoo.

-Steve


On 3/16/2010 5:05 PM, Ian Hickson wrote:
> On Tue, 3 Nov 2009, Brian Kuhn wrote:
>    
>> In section
>> http://www.whatwg.org/specs/web-apps/current-work/#attr-script-async, it
>> says:
>>
>> *Fetching an external script must delay the load event of the element's
>> document until the task that is queued by the networking task source
>> once the resource has been fetched (defined above) has been run.*
>>
>> Has any thought been put into changing this for async scripts?  It seems
>> like it might be worthwhile to allow window.onload to fire while an
>> async script is still downloading if everything else is done.
>>      
> On Fri, 6 Nov 2009, Brian Kuhn wrote:
>    
>> It seems to me that the purpose of async scripts is to get out of the
>> way of user-visible functionality.  Many sites currently attach
>> user-visible functionality to window.onload, so it would be great if
>> async scripts at least had a way to not block that event.  It would help
>> minimize the affect that secondary-functionality like ads and web
>> analytics have on the user experience.
>>      
> On Wed, 10 Feb 2010, Jonas Sicking wrote:
>    
>> I'm concerned that this is too big of a departure from how people are
>> used to<script>s behaving.
>>
>> If we do want to do something like this, one possibility would be to
>> create a generic attribute that can go on things like<img>,<link
>> rel=stylesheet>,<script>  etc that make the resource not block the
>> 'load' event.
>>      
> On Thu, 11 Feb 2010, Steve Souders wrote:
>    
>> I just sent email last week proposing a POSTONLOAD attribute for
>> scripts.
>>      
> On Thu, 11 Feb 2010, Jonas Sicking wrote:
>    
>> Though what we want here is a DONTDELAYLOAD attribute. I.e. we want
>> load to start asap, but we don't want the load to hold up the load
>> event if all other resources finish loading before this one.
>>      
> On Fri, 12 Feb 2010, Brian Kuhn wrote:
>    
>> Right.  Async scripts aren't really asynchronous if they block all the
>> user-visible functionality that sites currently tie to window.onload.
>>
>> I don't know if we need another attribute, or if we just need to change
>> the behavior for all async scripts.  But I think the best time to fix
>> this is now; before too many UAs implement async.
>>      
> On Fri, 12 Feb 2010, Nicholas Zakas wrote:
>    
>> To me "asynchronous" fundamentally means "doesn't block other things
>> from happening," so if async currently does block the load event from
>> firing then that seems very wrong to me.
>>      
> On Fri, 12 Feb 2010, Steve Souders wrote:
>    
>> ASYNC should not block the onload event. Thinking of the places where
>> ASYNC will be used, they would not want onload to be blocked.
>>      
> On Sat, 13 Feb 2010, Darin Fisher wrote:
>    
>> I don't know... to me, "asynchronous" means completes later.
>> Precedence: XMLHttpRequest.
>>      
> On Sat, 13 Feb 2010, Boris Zbarsky wrote:
>
>    
>> [...] my real worry about making any loads that don't block onload:
>> would web developers expect them to?
>>      
> On Sat, 13 Feb 2010, Brian Kuhn wrote:
>    
>> FWIW, loading scripts asynchronously with the "Script DOM Element"
>> approach does not block window.onload in IE.  In Chrome and Safari, the
>> downloading blocks, but execution doesn't.  In Firefox and Opera,
>> downloading and execution blocks.
>>
>> So, it's pretty hard to say what web developers would expect with async
>> scripts.  I know that they will like having things like ads and
>> analytics not block window.onload though.  At the very least, we need
>> that ability to make that happen.
>>      
> On Sat, 13 Feb 2010, Jonas Sicking wrote:
>    
>> Yeah, my big concern is "what do developers expect". Having an explicit
>> attribute for not blocking onload definitely follows the path of least
>> surprise. Though having an explicit attribute does give Steve more
>> things to evangelize, i.e. it'll probably lead to more pages firing
>> onload later than they could.
>>      
> On Sat, 13 Feb 2010, Darin Fisher wrote:
>    
>> The thing is, almost all subresources load asynchronously.  The load
>> event exists to tell us when those asynchronous loads have finished.
>> So, I think it follows that an asynchronous resource load may reasonably
>> block the load event.  (That's the point of the load event afterall!)
>>      
> I've changed the spec to fire 'DOMContentLoaded' without waiting for the
> async scripts, so that if you need this you can just listen for that event
> instead of 'load'. 'load' still waits for all scripts. 'DOMContentLoaded'
> still waits for deferred scripts. As far as I can tell this handles all
> the above (still makes sense, still consistent with the way other 'load'
> events work, but still lets you do things without waiting).
>
>
> On Wed, 30 Dec 2009, David Bruant wrote:
>    
>> The "6.8.1 Client identification" starts with an explanation dealing
>> with browser-specific bugs and limitation ("browser-specific features"
>> are missing, aren't they ?) that Web authors are forced to work around.
>>      
> "Browser-specific features" should be featured-tested, not version-tested.
>
>
>    
>> A very interesting project dealing with these browsers specific
>> implementations is TestSwarm : http://testswarm.com/
>>
>> As you may notice, the web browsers are classified this way :
>> 1) Operating system
>> 2) Web Browser (the equivalent of the current "window . navigator . appName")
>> 3) Version (the equivalent of the current "window . navigator . appVersion")
>>
>> In my opinion, the TestSwarm approach is relevant because a user agent with
>> the same "appName" and "appVersion" can have OS-specific bugs. However, in the
>> NavigatorID interface, there is currently no way to detect the operating
>> system. The current way to detect the operating system is to use the userAgent
>> String. However, this can be freely overridden in some browsers. As a
>> consequence, this string cannot be relied at all for OS detection.
>>
>> To allow OS-specific bugs detection, a property could be added to the
>> NavigatorID interface.
>> window.navigator.operatingSystem ? opSysName and opSysVersion ? Name, SubName,
>> Version, SubVersion ? Something even more detailled ?
>> It would certainly also be the role of the spec to list the operating systems,
>> specify the concordant strings and/or give a rule for future and unknown
>> version and operation systems.
>>      
> These strings could also be freely overridden in some browsers, so I don't
> see why it would be any more reliable.
>
> I'd rather not add yet more features to this particular object. We should
> discourage this kind of thing, not encourage it.
>
>
> On Wed, 20 Jan 2010, David Flanagan wrote:
>    
>> I'm trying to understand the async and defer attributes of the script
>> tag. Unfortunately, since script execution is so intimately tied up with
>> HTML parsing, section 4.3.1 is particularly hard to make sense of. I've
>> got 3 questions, and 3 suggested clarifications to the spec. Thanks to
>> anyone who can explain these!
>>
>> First, my questions.  Are the following three statements correct?  (I'm
>> only concerned with<script>  tags that actually appear in a document,
>> not those inserted or emitted (via document.write()) by another
>> script.):
>>
>> 1) Scripts without async or defer attributes are executed in the order
>> in which they appear in the document.  They are executed synchronously,
>> which means that the parser must stop parsing the document while they
>> run.
>>      
> Mostly. There are ways to make things happen out of apparent order, e.g.
> using document.write() and appendChild(). I'd rather not try to explain
> this in detail (in the spec, for authors) because it's likely to be wrong,
> and it's likely to cause implementors to ignore the real spec and instead
> read the author-facing description. We've already seen this happen
> multiple times for the green DOM intro boxes.
>
>
>    
>> 2) Scripts with the defer attribute, but without the async attribute are
>> executed in the order in which they appear in the document, but their
>> execution is deferred until the document has finished parsing. All these
>> scripts will execute before DOMContentLoaded and the load event are
>> fired.  A deferred script can assume that the entire DOM tree has been
>> constructed and is ready for manipulation--these scripts do not
>> generally need to register an onload event handler. A call to
>> document.write() within a deferred script will blow away the current
>> document and begin a new one.
>>      
> Sounds right.
>
>
>    
>> 3) Scripts with the async attribute are executed as their script content
>> becomes available over the network, with no guarantee that they will be
>> executed in the order in which they appear in the document.  The only
>> guarantee is that these scripts will run before the DOMContentLoaded or
>> load events are fired. Document parsing may or may not have completed
>> when an async script is run, and a call to document.write() from an
>> async script will have unpredictable behavior. Though the order of
>> execution of async scripts is not predictable, the scripts will always
>> appear to run in some serial order without concurrent execution.
>>      
> I just changed the DOMContentLoaded event to fire before the async events.
> Other than that it is correct.
>
>
>    
>> Next, I suggest that the following things in the spec be clarified:
>>
>> 1) After describing the async and defer attributes, the spec promises:
>> "The exact processing details for these attributes are described below."
>> I take this to mean "below, somewhere in section 4.3".  In fact,
>> however, the exact processing details are scattered throughout the spec,
>> and understanding the attributes requires understanding section 9, I
>> think. It would be nice to note this.
>>      
> Done.
>
>
>    
>> 2) The last sentence of this paragraph:
>>
>>      
>>> The second is a flag indicating whether the element was
>>> "parser-inserted". Initially, script elements must have this flag
>>> unset. It is set by the HTML parser and is used to handle
>>> document.write() calls.
>>>        
>> made me think that the "parser-inserted" flag would only be set to true
>> for scripts that were emitted through document.write() calls.  That is,
>> I thought that the parser-inserted flag would be set only in unusual
>> cases rather than in the most common case.  This section should explain
>> the meaning of the parser-inserted flag. Instead it describes one of the
>> purposes of the flag, but that purpose is different than the purpose for
>> which it is used in this section.
>>      
> I've tried to make it less confusing.
>
>
>    
>> 3) The algorithm for "running a script" adds scripts to "the list of
>> scripts that will execute as soon as possible".  And 9.2.6 spins the
>> event loop until this list is empty.  But I don't see anything in the
>> spec that removes items from this list.  That seems like an error in the
>> spec, not just a confusing bit.
>>      
> Oops. Fixed.
>
>
>    
>> Furthermore, the fact that this mechanism is specified as a "list"
>> rather than as a "set" implies some kind of sequential execution of the
>> scripts.  But I don't think any sequence is meant here.
>>      
> Fixed.
>
>
> On Mon, 8 Feb 2010, Steve Souders wrote:
>    
>> I have some comments and questions about the ASYNC and DEFER attributes
>> of the SCRIPT tag based on reading this document:
>> http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html
>> <http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html#script>
>>
>> 1. "If neither attribute is present, then the script is fetched and executed
>> immediately, before the user agent continues parsing the page."
>>      Thankfully, newer browsers are downloading scripts in parallel with other
>> resources. I presume the way this is done is they launch a request for a
>> script and continue to do "speculative" parsing looking for other resources
>> (images, stylesheets, other scripts, etc.) and launch those requests. But this
>> nice feature seems to be in conflict with the above text because the browser
>> continues parsing (albeit speculatively) before the script is executed. It
>> would be good to mention this optional behavior here, something along the
>> lines of browsers may want to do speculative parsing, but shouldn't create DOM
>> elements, etc. - only kickoff HTTP requests.
>>      
> The spec says:
>
> # Conformance requirements phrased as algorithms or specific steps may be
> # implemented in any manner, so long as the end result is equivalent.
>
> # For performance reasons, user agents may start fetching the script as
> # soon as the attribute is set, instead, in the hope that the element will
> # be inserted into the document. Either way, once the element is inserted
> # into the document, the load must have started. If the UA performs such
> # prefetching, but the element is never inserted in the document, or the
> # src attribute is dynamically changed, then the user agent will not
> # execute the script, and the fetching process will have been effectively
> # wasted.
>
> ...which is intended to allow this. I suppose I could change the last
> paragraph quoted above to allow downloads even before elements are
> created, if you think that would be better.
>
>
>    
>> 2. "If one or both of the defer and async attributes are specified, the src
>> attribute must also be specified."
>>      It should be possible to specify DEFER without a SRC. The use case is a
>> page that has a sequence of SCRIPTs (with and without a SRC attribute) all of
>> which need to execute in order, but should do so without blocking the parser.
>> This happens a lot with ads, widgets, and analytics. A workaround is to use
>> callbacks to daisy-chain the calling sequence, but the complexity will lead
>> most 3rd party snippet providers to default to a normal SCRIPT tag (without
>> DEFER or ASYNC) resulting in blocking the parser and slow pages. It's
>> especially annoying for web site owners to have 3rd party content slowing down
>> their pages and blocking the content they've created.
>>      This appears to be a recent change perhaps prompted by Jonas Sicking's
>> comments that Mozilla found many web sites that specified DEFER without a SRC
>> and then called document.write (which pretty clearly indicates the developer
>> didn't mean to specify DEFER). If that's the motivation for this restriction,
>> we need to either find an alternative syntax or go ahead and allow DEFER
>> without SRC. Finding an alternative is the worse alternative (DEFER has the
>> exact behavior we want, so creating something with a different name that
>> behaves just like DEFER is confusing). If we do move forward with allowing
>> DEFER without SRC, then we need to specify what happens if it contains
>> document.write so that the entire document isn't overwritten. (I believe this
>> is addressed in section 3.5.) There's no good way to make DEFER do what it
>> should and have those pages who are using DEFER incorrectly work the way they
>> do now. With this path, at least those pages will have their content appear at
>> the bottom and not wipe out the entire page.
>>      
> The reason for the spec being the way it is is indeed as you describe
> (well, IIRC it has more to do with innerHTML than document.write(), but
> it's similar). However, the problem is non-trivial. Getting compatibility
> with legacy content while supporting defer="" on<script>  without src=""
> appears to me to be hugely complicated.
>
> It seems pretty easy to work around this limitation by just having
> callbacks in the code, though, so I don't really see this as a huge
> problem.
>
>
>    
>> 3. "[the 'parser-inserted' state] is set by the HTML parser and is used to
>> handle document.write() calls."
>>      In what way is this used to handle document.write() calls? Is it for
>> handling additional SCRIPTs added via document.write, or to make
>> document.write itself have different behavior? The answer should be added to
>> the spec somewhere. I searched for .write in this document and didn't find an
>> explanation.
>>      
> I removed that comment. To answer your question, though, it's used to
> ensure that script run in the right order which affects the insertion
> point which affects the way document.write() works.
>
>
>    
>> 4. "If the element has a src attribute, [snip] the specified resource must
>> then be fetched, from the origin of the element's Document."
>>      If the script has DEFER, the request should not start until after parsing
>> is finished. Starting it earlier could block other (non-deferred) requests due
>> to a connection limit or limited bandwidth.
>>      
> Browsers can prioritise when scripts are loaded, but I think it would be a
> mistake to disallow browsers from fetching scripts earlier than load. (To
> start with I think they're ignore the requirement.)
>
>
>    
>> 5. I don't see any rules for the order of executing scripts added to the
>> "list of scripts that will execute when the document has finished
>> parsing" and the "list of scripts that will execute as soon as
>> possible". DEFER scripts should execute in the order they appear in the
>> list. ASYNC scripts should be executed as soon as the response is
>> received.
>>      
> The rules are implicit in how the lists (the list and the set, now) are
> processed.
>
>
> On Wed, 10 Feb 2010, Steve Souders wrote:
>    
>> In the current text, it says "must then be fetched". In my suggestion I
>> say "should not start until after parsing". Saying "should" instead of
>> "must" leaves the opening for browsers that feel they can fetch
>> immediately without negatively impacting performance.
>>      
> "Must be fetched" means it runs the "Fetch" algorithm, which explicitly
> says that the download happens "at a time convenient to the user and the
> user agent".
>
>
> On Wed, 10 Feb 2010, Jonas Sicking wrote:
>    
>> Instead, if the use cases are strong enough, I think we need to
>> introduce another mechanism for delaying a<script>  to get loaded until
>> after the 'load' event has fired. I think it's an interesting idea to
>> add a 'postonload' attribute to all resources, such as<script>,<img>
>> and<link rel=stylesheet>  (though the maybe the name could be better).
>>      
> On Mon, 8 Feb 2010, Steve Souders wrote:
>    
>> I'd like to propose the addition of a POSTONLOAD attribute to the SCRIPT
>> tag.
>>
>> The behavior would be similar to DEFER, but instead of delaying
>> downloads until after parsing they would be delayed until after the
>> window's load event. Similar to DEFER, this new attribute would ensure
>> scripts were executed in the order they appear in the document, although
>> it could be combined with ASYNC to have them execute as soon as the
>> response is received.
>>      
> This idea is interesting, but I think it's better for us to wait until
> we've seen what browsers do with async="" before adding yet another
> feature to<script>. If we add too much at once, browsers will have no
> hope of implementing it all correctly. :-)
>
> Given that it is possible to do this from script, how common is it for
> people to do it from script? If it's very common, that would be a good
> data point encouraging us to do this sooner rather than later.
>
>
> On Thu, 11 Feb 2010, Mathias Sch?fer wrote:
>    
>> In a JavaScript tutorial, I wanted to explain what DOMContentLoaded
>> actually does. But the tests I made revealed that there isn't a
>> consistent behavior across browsers with regard to stylesheets. In fact,
>> it's a total mess. These are the results of my tests:
>>
>> http://molily.de/weblog/domcontentloaded
>>
>> Please have a quick look at these findings (you can skip the
>> introduction part). My questions are:
>>
>> 1. Am I right that HTML5 will standardize Opera's pure DOMContentLoaded
>> model, never waiting for stylesheets? My assumption is that this will
>> break compatibility with the current Gecko and Webkit implementations.
>>      
> Scripts can block the whole parser until style sheets have loaded, which
> implicitly means the script will wait too, but other than that, yes, the
> spec doesn't wait for style sheets for DOMContentLoaded.
>
>
>    
>> 2. Does the HTML5 parser specify that external stylesheets defer
>> external script execution? As far as I understand the specs, it doesn't.
>>      
> It does, unless I made a mistake.
>
>
>    
>> In Gecko and IE, the loading of stylesheets also defers the execution of
>> subsequent *inline* scripts. I haven't found a rule for that in the
>> HTML5 parsing algorithm either. Does it conform to the specs, is it
>> against the rules or a legitimate extension which is not covered by
>> HTML5?
>>      
> The spec does require that. Search for "a style sheet blocking scripts".
>
>
> On Wed, 10 Feb 2010, Boris Zbarsky wrote:
>    
>> http://www.whatwg.org/specs/web-apps/current-work/multipage/scripting-1.html#running-a-script
>> step 8 the cases that talk about "a style sheet blocking scripts"
>> specify this.
>>
>> I really wish those steps had individual IDs, and so did the cases
>> inside them.  It'd make it a lot easier to link to them!
>>      
> Added. Let me know if you think anything else needs them. (I don't want to
> automatically add them to every paragraph because the resulting bloat is
> excessive.)
>
>
> On Thu, 11 Feb 2010, Mathias Sch?fer wrote:
>    
>> The question is: Is a normal external script ?parser-inserted? or not?
>> I assume the flag to be false, since that?s the default value and I
>> found ?parser-inserted? to be true for XML parsing only
>> (#parsing-xhtml-documents). Correct?
>>      
> I've changed the comment near the definition of the term to be clearer. I
> hope that helps.
>
>
>    
>> Just to translate from HTML5 speak into my own words. I?ve got ...
>>
>> <link rel="stylesheet" href="...">
>> <script src="..."></script>
>>
>> ... and I would like to step through the parsing algorithm. This is my
>> understanding so far:
>>
>> 1. Run the script (#parsing-main-incdata, case ?An end tag whose tag
>> name is "script"?)
>>      
> You missed the<script>  start tag processing, which sets the
> "parser-inserted" flag:
>
>     http://www.whatwg.org/specs/web-apps/current-work/multipage/tokenization.html#scriptTag
>
>
>    
>> That means, inline script execution should also wait for stylesheets to
>> load. Am I right in this reading?
>>      
> Yes.
>
>    
Received on Wednesday, 17 March 2010 14:44:53 UTC

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