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

I've had a couple of recent examples where this would make a positive
difference:

1. Site with JS at the bottom of the page with images served from the same
hostname.

In browsers like Chrome that prioritise the download of resources based on
type they this doesn't cause an issue but in IE for example, the images
were consuming all the TCP connections leading to delay in downloading the
JS and hence DCL firing.

Sharding the images onto a separate hostname, resolves the blocking issue
but then they're still competing for bandwidth which may or may not be a
problem depending on the connection.

Of course prioritised downloading is probably the real answer here but are
other browsers likely to implement it quicker than defer could be adopted?


2. Holiday site with image carousel on each destination page, carousel is
hidden by default

All images were loaded from a separate hostname but carousel ones were
downloaded all the time and delayed the download of over visible images

There are work arounds to both these issues e.g. CDN's, use data-
attributes and JS to defer download etc. but my preference is an approach
that gives the UA hints and allows it to make appropriate choices. (It also
means content can be indexed without robots needing to execute JS)

I know this discussion is largely focused on images right now but there are
other areas where better hinting of priorities might help performance e.g.
a way of declaratively deferring loading of 3rd party scripts until a given
event fires e.g. onload

There's a section in Andy Hume's talk from The Responsive Day out (
http://www.besquare.me/session/responsive-page-load/) where he talks about
The Guardian's experiments at splitting the page load into "content,
enhancements and leftovers", with the loading of the last two fired by DCL
and onload respectively.

I guess the question is what makes sense for browsers to handle and what
should be left to JS.

Andy


On 14 April 2013 18:52, Jake Archibald <jakearchibald@google.com> wrote:

> Hi,
>
> I know this group is looking at resource priorities, but I'm interested in
> taking it a big further for images, and perhaps other visual media such as
> video.
>
> # Problems I'd like to solve:
>
> Developers are using hacks or ending up with unnecessary downloads when
> they use JavaScript to change the src of an image, but want to provide a
> non-JS fallback (see https://www.w3.org/Bugs/Public/show_bug.cgi?id=17842).
> Although this particular case will be solved with something like srcset,
> there are other cases like conditionally using an image format with limited
> support.
>
> Developers are using hacks to lazy-load images that are likely to be
> outside the viewport initially. These hacks use heavy scroll listeners and
> don't take the connection type into account.
>
> Images within elements that are never shown are downloaded.
>
> # "defer" behaviour:
>
> Images with 'defer' MUST NOT download while they aren't in a document, or
> their calculated 'display' style property is 'none'.
>
> The download state of images with 'defer' MUST NOT delay the 'load' event
> of the window.
>
> Images with 'defer' MAY download once they are in the document, and their
> calculated 'display' style property is not 'none'.
>
> Images with 'defer' MUST download once they are in the document, and their
> calculated 'display' style property is not 'none', and any part of the
> image would be visible within the viewport. If visibility within the
> viewport is uncertain (due to element size determined by image dimensions),
> the image MUST download.
>
> There's been a lot of discussion on
> https://www.w3.org/Bugs/Public/show_bug.cgi?id=17842, I'll try and sum up
> the pros & cons but will ping Ilya to add anything I may have missed.
>
> # Pros
> * Imagery in hidden elements that may never be seen by the user aren't
> downloaded
> * JS can modify the src of images before their original src downloads, by
> initially hiding the image with CSS
> * The browser gets more freedom in regards to image downloading, allowing
> it to do the best thing given the user's connection type & interaction. Eg:
> ** If making a connection is cheap & quick (eg, fast wifi), the browser
> may delay downloading imagery until it's in the viewport, which can avoid
> unnecessary downloads way below the fold
> ** If making a connection is expensive & slow (eg, 3g), the browser may
> download all imagery that isn't calculated display:none to avoid waking the
> radio up later and using up battery
>
> # Cons
> * When creating images with js, you'd need to set .defer to true before
> setting .src, else the image would download straight away
>
> I don't see this as a big problem, devs know the ordering matters, you
> already have to set src after load events
>
> * Deferred images cannot be downloaded early by preloaders unless they
> calculate page styles & images that are at the top of the page may download
> later as they need to wait for style calculation to know if they should
> download or not
>
> Not a big deal. Images are low priority downloads anyway compared to JS &
> CSS so waiting for layout isn't a problem. Also, primary imagery shouldn't
> use this attribute.
>
> * Firing the load event before all images download may skew metrics
>
> By putting 'defer' on an image you're saying it isn't required to consider
> the page loaded. I don't think this is an issue.
>
> * Lazy-loading isn't good on mobile connections
>
> The browser isn't required to lazy-load. It can make the choice based on
> the connection, which is far better than the hacky JavaScript developers
> are using to currently perform lazy-loading.
>
> * Developer can't indicate they'd rather have aggressive lazy-loading, to
> save CDN costs
>
> Feature, not a bug. Would rather the browser did what's best for the
> connection type. The developer could always display:none the images, and
> use a scroll listener to show them as they come into view, but I wouldn't
> want to make that any easier.
>
> Cheers,
> Jake.
>