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Re: Spec behavior for img.src sets is not quite web-compatible

From: Ian Hickson <ian@hixie.ch>
Date: Mon, 11 Jun 2012 21:44:45 +0000 (UTC)
To: Boris Zbarsky <bzbarsky@MIT.EDU>
cc: HTML WG <public-html@w3.org>
Message-ID: <Pine.LNX.4.64.1206111923580.378@ps20323.dreamhostps.com>
On Fri, 16 Mar 2012, Boris Zbarsky wrote:
>
> Consider the following testcase:
> 
> <!DOCTYPE html>
> <img
> src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAUCAIAAAALACogAAAAAXNSR0IArs4c6QAAAAlwSFlzAAALEwAACxMBAJqcGAAAABtJREFUKM9jFBB4z0AKYGIgEYxqGNUwqoGeGgBbDwE3IUzUhQAAAABJRU5ErkJggg==">
> <img
> src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAIAAAD8GO2jAAAAAXNSR0IArs4c6QAAAChJREFUSMftzUEBAAAEBLCjf2dK8NsKrJLJp84zgUAgEAgEAoFAcGUBocYBP+pqhN4AAAAASUVORK5CYII=">
> <script>
> window.onload = function() {
>   var img = document.querySelector("img");
>   img.src =
> "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAIAAAD8GO2jAAAAAXNSR0IArs4c6QAAAChJREFUSMftzUEBAAAEBLCjf2dK8NsKrJLJp84zgUAgEAgEAoFAcGUBocYBP+pqhN4AAAAASUVORK5CYII=";
>   alert(img.width + " " + img.height);
> }
> </script>
> 
> Every non-Gecko UA alerts "32 32".  Gecko used to do the same thing before
> Firefox 4.  At that point we switched to doing what the spec calls for here,
> which is that the src set starts an async fetch and the image size does not
> update until the fetch completes.
> 
> Unfortunately, this does not seem to be web-compatible.  People expect their
> preloaded images to synchronously update the width and height when src is set
> to them.
> 
> I'm probably going to change Gecko to do that.  I have no idea how to spec it,
> though, because the spec doesn't have any concept of images being preloaded,
> and I'm not quite sure it should.  :(
> 
> -Boris
> 
> P.S.  The behavior is the same with non-data: URIs; the data: URIs just made
> it easier to construct a standalone testcase.

I have tried to specify this. I'm not sure I got the timing of load events 
exactly right; if not, let me know.

Testing interop on this will be... difficult, since UAs are basically 
allowed to do whatever they want.

http://html5.org/tools/web-apps-tracker?from=7127&to=7128

-- 
Ian Hickson               U+1047E                )\._.,--....,'``.    fL
http://ln.hixie.ch/       U+263A                /,   _.. \   _\  ;`._ ,.
Things that are impossible just take longer.   `._.-(,_..'--(,_..'`-.;.'
Received on Monday, 11 June 2012 21:46:00 GMT

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