The reason is explained here: https://www.w3.org/Bugs/Public/show_bug.cgi?id=18455
I've added a comment to the spec bug since it breaks some layout tests.
Created attachment 157112 [details] WIP. Breaks existing layout tests
Created attachment 158678 [details] stop load and error events
A bug 93920 was resolved. Now we can stop 'load' and 'error' events at shadow boundaries as the spec says.
Comment on attachment 158678 [details] stop load and error events Clearing flags on attachment: 158678 Committed r125744: <http://trac.webkit.org/changeset/125744>
All reviewed patches have been landed. Closing bug.