[whatwg] Deferring image load

[Bjartur Thorlacius]

On 2/13/12, Gray Zhang <otakustay at gmail.com> wrote:
> We would like to present an authoring difficulty with regard to showing
> images on the Web with limited bandwidth, when deferring loading of certain
> or all images are preferable. We have some vague ideas about what
> browser/markup solutions instead of script solutions might look like, but
> we are more interested in having the problems solved.
>  Scenario
>
>    1. On a product description page of a shopping site, there are several
>    *main* pictures of the product, along with about twenty or so camera
>    pictures of the product taken from different angles. When the HTML is
>    parsed, browsers by default simultaneously start downloading all images,
>    potentially making some of the *main* ones invisible.
Hmm. So you request a way to declare which images are important, and
wich are not?

>    2. On an album page where hundreds of pictures are expected to be shown,
>    it is often required that pictures currently in a user's screen should
>    appear as fast as possible. Loading of a picture outside the screen can
> be
>    deferred to the time that the picture enters or is about to enter the
>    screen, for the purpose of optimization user experience.
This seems like something interactive user agents should implement.

>    3. For a site with limited bandwidth on the server side, it is
>    preferable to minimize the amount of data transferred per each page view.
>    70% of the users only read the first screen and hence pictures outside
> the
>    first screen don't need to be downloaded before the user starts to scroll
>    the page. This is to reduce server-side cost.
>
>    1. the @defer attribute on <img> and <iframe>
>    2. the similar @deferpolicy attribute with value a combination of
> visible,
>    network-idle and render-idle
>    3. a global switch as a http header or an attribute on html to switch
>    UAs image loading from "obtain images immediately" to "obtain on demand"
> or
>    vice versa.
Would this not depend equally on factors such as whether the user
agent would download the images over a metered connection?