My personal experience is different. We found that using img tags is not that 
reliable. Especially in Firefox we recently saw some problems. Img tags in 
general have the disadvantage that the amount of data that can be set is rather 
limited. While this obviously should be kept as small as possible, the 
information available via resource timing will increase the amount of data that 
gets sent.

Is there a way we can integrate this into a W3C test suite to check how 
different browsers behave in this case

// Alois

From: Andy Davies <<>>
Date: Sunday, February 17, 2013 11:12 PM
To: Maciej Stachowiak <<>>
Cc: "<>" 
<<>>, Anne van Kesteren 
<<>>, Web Applications Working Group WG 
Subject: Re: Beacon API
Resent-From: "<>" 
Resent-Date: Sunday, February 17, 2013 11:13 PM

On 16 February 2013 05:25, Maciej Stachowiak 
<<>> wrote:

BTW as far as I know the best current nonblocking technique to phone home on 
unload is to create an <img> in your unload handler pointing to the ping URL, 
this will result in reliable delivery without blocking at least in IE and 
WebKit-based browsers. I've found it hard to convince even knowledgable web 
developers to use this technique or <a ping> over synchronous XHR, even sites 
that are otherwise willing to do Safari-specific optimizations. I am not sure 
why sync XHR in unload is so tantalizing.

I asked Phillip Tellis (creater of Boomerang) about this and his experience is: 
(copied because he can't post to the list)

"A beacon may be sent using a GET request via an Image object in a listener for 
either the onunload,the onbeforeunload or the onpagehide events.  No browser 
blocks if this is sent via the onbeforeunload event, however Opera does not 
support this event.  Firefox is the only browser that blocks for Image requests 
in the onunload event.

Additionally, attaching a listener to the onunload event affects Page Cache in 
most browsers, so one should avoid that in any event.

The solution we've come up with for boomerang (a general purpose beaconing 
solution primarily used for web performance beacons) is to attach a listener to 
the beforeunload event and either the pagehide or unload, depending on which is 
available (pagehide is checked first).  The first of these events to fire sends 
the beacon out and removes the listener from the other.  This has allowed us to 
fire beacons on every page unload without affecting unload time and without 
beacon loss."

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