Recently 'A List Apart' posted
an article about the new X-UA-Compatible switch that Microsoft will be implementing in Internet Explorer 8.
In short, you can target versions of the IE renderer with the switch; you can say: I want this page to be rendered like IE7 would render it and then IE8 (and IE9 and IE10 presumably) would render it similarly to IE7.
Now this provoked a lot of reactions, and it got me thinking about one of the most painful subjects among developers (PHP and in general):
backward compatibility.