Async script loading of 3rd party JS
It is now accepted knowledge that 3rd party JavaScript should be loaded asynchronously. There are various benefits with respect to speed and limited interference with loading the embedding page, but the main benefit is that in scenarios like a couple weeks ago when Facebook went down, a timeout of a 3rd party script tag, does not automatically take down your page as well.

The classic async embed code looks something like this (I use +1 because that is what I work on)
<script type="text/javascript">
  (function() {
    var po = document.createElement('script'); po.type = 'text/javascript'; po.async = true;
    po.src = 'https://apis.google.com/js/plusone.js';
    var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(po, s);
  })();
</script>

Now, there is a new HTML5 hotness out there that is much simpler:
<script src="https://apis.google.com/js/plusone.js" async></script>

Browsers do not execute JS until all stylesheets that are referenced above it and all scripts above it in the DOM have been loaded and executed. That means that will the old-style embed code, your browser likely won't even start downloading the script until very late in the load process of a given page.

Modern browsers have look ahead parsers that start downloading resources like stylesheets and scripts long before they may be able to actually run them, essentially just scanning the HTML and looking for everything that may be useful in the future. This is where the new hotness wins big: Being a normal script tag, the pre-scanning parser can see it and start downloading the script early on.

So we want the async attribute! BUT, of course, there is a catch: IE<10 does not support it (I know, surprise, surprise). All stable IEs will simply ignore the attribute and load just fine, just like a normal script tag, but they won't get the benefit of "crash-resistance" should the 3rd party site go down. So, my general question: Would you, for your site, want to give modern browsers faster loading and have IE9 and below suffer from increased risk of downtime?
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