[whatwg] ProgressEvents for Images

[Hans Muller]

A group of us at Adobe has been looking into adding support for ProgressEvents to images.  The overall goal is to simplify image download progress reporting by supporting roughly the same progress events as XHR and the File API for image elements.   For example one could connect an image to a progress element like this:

<img id="image" src="sample.jpg"
    onloadstart="showProgressBar()"
    onprogress="updateProgressBar(event)"
    onloadend="hideProgressBar()"/>

Developers have taken various tacks to enable progress reporting, for example in some cases XHR can be used to download image files.  Max Vujovic just published a blog about the practicalities of doing so: http://blogs.adobe.com/openweb/2012/01/13/html5-image-progress-events/.  We think it would be preferable to provide support for image progress events directly.

We're working on a prototype implementation for WebKit and have filed a bug that explains what we're up to in a little more detail: https://bugs.webkit.org/show_bug.cgi?id=76102).

It's probably worth pointing out that the beforeload event, which is currently under discussion, addresses a different use case.  Our proposal is intended to enable applications to give the user feedback about image download progress, it's not intended to enable security or efficiency by preemptively blocking or transforming image downloads.

We'd appreciate feedback on this proposal.