Unclear "iframe load event steps" for initial load of about:blank in an iframe #490
Description
The spec says that an iframe with no src attribute on first time through process the iframe attributes will queue a task to run iframe load event steps, which dispatches the load event at the iframe element.
For the following test hosted on joshmatthews.net:
<script>var sync_event1 = true;</script>
<iframe id="first" onload="console.log('first - onload from inline: ' + document.querySelector('#first').contentWindow.location + ' ' + sync_event1)"></iframe>
<script>
var iframe1 = document.querySelector('#first');
iframe1.addEventListener('load', function() {
console.log('first - onload during script execution: ' + iframe1.contentWindow.location + ' ' + sync_event1);
});
sync_event1 = false;
</script>
<script>var sync_event2 = true;</script>
<iframe id="second" onload="console.log('second - onload from inline: ' + document.querySelector('#second').contentWindow.location + ' ' + sync_event2)"></iframe>
<script>
var iframe2 = document.querySelector('#second');
iframe2.addEventListener('load', function() {
console.log('second - onload during script execution: ' + iframe2.contentWindow.location + ' ' + sync_event2);
});
iframe2.src = "http://joshmatthews.net";
sync_event2 = false;
</script>I get this output from browsers -
Firefox, Edge:
first - onload from inline: about:blank false
first - onload during script execution: about:blank false
second - onload from inline: http://www.joshmatthews.net/ false
second - onload during script execution: http://www.joshmatthews.net/ false
Chrome, Safari:
first - onload from inline: about:blank true
second - onload from inline: about:blank true
second - onload from inline: http://www.joshmatthews.net/ false
second - onload during script execution: http://www.joshmatthews.net/ false
Specifically, Firefox appears to skip the initial load event for about:blank if the iframe's src is changed before the queued task executes, while Chrome's load event for the initial about:blank load is dispatched synchronously.
What is the intended behaviour here?