Re: <img src="..." defer>

Hi, Jatinder.

Browsers are built to work best across *most* websites. But that doesn't
mean their behavior works best across *all* websites. When the default
behavior hurts a website's performance, it's good to think about what
alternatives exist for site owners.

In this example Airbnb would be better off if the huge image was downloaded
in parallel with the JS & CSS. That would slightly delay when the JS & CSS
complete, but would greatly accelerate when the huge image is rendered.
Without a way to promote the huge image's priority in markup, I would
recommend that Airbnb load the huge image using JS at the top of HEAD
(above all the scripts and stylesheets), and then swap it in on onload.
That's not the end of the world - telling websites to load critical images
dynamically at the top of HEAD. But it's not as clean as using markup.

I mainly wanted to clarify that in fact the behavior being proposed,
limited to just specifying 'delayload' for unnecessary images, does not
produce the best performance in this case.

-Steve


On Wed, May 22, 2013 at 5:15 PM, Jatinder Mann <jmann@microsoft.com> wrote:

>  We had spent some time discussing this issue at the *W3C Workshop for
> Performance. *Even if a developer wanted to specify that a single image
> be the highest priority download, I don’t believe most major browser
> vendors would respect that priority order over CSS and synchronous
> JavaScript. Seeing these are hints to the browser, we wanted to avoid
> having developers try to specify different classes of priority numbers for
> resources. For most web developers, it feels like the easiest thing is to
> specify resources that they know aren’t necessary for above the fold
> visuals.****
>
> ** **
>
> In your example, the best approach for developers is to mark all
> unnecessary resources for the above the fold visuals as ‘delayload’. This
> way all the important images, CSS, and JavaScript will be downloaded in the
> order the browser feels is most optimal. For example, a user agent may
> download in the following order: root document, CSS in document order,
> JavaScript in document order, everything else in document order,
> delayloaded resources in document order.****
>
> ** **
>
> Thanks,****
>
> Jatinder****
>
> ** **
>
> *From:* Steve Souders [mailto:souders@google.com]
> *Sent:* Wednesday, May 22, 2013 12:36 PM
> *To:* Jake Archibald
> *Cc:* Jatinder Mann; public-web-perf@w3.org
>
> *Subject:* Re: <img src="..." defer>****
>
> ** **
>
> SHORT: We need the foil of "defer" so that image download priority is
> INcreased.****
>
> ** **
>
> LONG: ****
>
> I love the idea of being able to defer images, but that leaves images with
> only these two states:****
>
>     - *deferred*****
>
>     - *not deferred*****
>
> ** **
>
> So I have two choices: my images get loaded very late, or they get loaded
> late (because scripts, stylesheets, and fonts have a higher download
> priority in some major browsers). ****
>
> ** **
>
> *What if I have an IMG that I want to load at a higher priority?! *****
>
> ** **
>
> Here's a great example. Airbnb has a huge IMG that fills the background of
> the entire page. It's the very first IMG in the document. It's obviously
> critical to the UX. But, because of Chrome priorities, it gets loaded too
> late. In these WPT results<http://www.webpagetest.org/video/compare.php?tests=130515_TD_QYX-r%3A1-c%3A0&thumbSize=200&ival=100&end=visual> this
> huge image (the first IMG in the document!) gets pushed out to request #12
> and doesn't show up until 3.4 seconds into the page load.****
>
> ** **
>
> Instead, it would be better if this IMG was downloaded with equal priority
> to scripts & stylesheets, or at least higher priority than any other image
> (including CSS images). But, even if "defer" gets adopted, we'll still have
> no way to promote this IMG to get it to appear earlier.****
>
> ** **
>
> We need the foil to "defer" so that image download priority is INcreased.*
> ***
>
> ** **
>
> -Steve****
>
> ** **
>
> ** **
>
> On Wed, May 22, 2013 at 8:25 AM, Jake Archibald <
> jakearchibald@chromium.org> wrote:****
>
>  On Tue, May 21, 2013 at 9:09 PM, Jatinder Mann <jmann@microsoft.com>
> wrote:****
>
>  Based on discussions with web developers, the attribute name ‘delay’
> doesn’t seem clear and can carry the wrong connotations that the attribute
> is slowing down the page load ****
>
>   …I recommend we instead use the attribute name ‘delayload’. This name
> is very clear that the attribute will delay loading the lower priority
> resource.****
>
>   ****
>
> I imagine developers will react the same to 'delayload' as they do to
> 'delay', although I'm just guessing.****
>
> ** **
>
> 'delayload' as in 'delay the load' sounds like the image may delay the
> loading of the page, when (in terms of the load event of the document) it
> does the opposite. ****
>
> ** **
>
> 'lazy' or 'lazyload' sounds more like it's only impacting the image,
> perhaps because it's an existing term and used frequently in relation to
> image loading.****
>
>  ****
>
>  Also, as XHRs can just be deprioritized through script and they are the
> only non-element on the list, I recommend we just remove them.****
>
>  ** **
>
> +1****
>
>  ** **
>