HTML, CSS, Javascript and more
Since IE8 you can force an IE8+ browser to handle a website like it is an IE7 or an IE8 browser. IE9 seems to emulate older browsers well, but I know at least one rendering bug and I’m wondering if some backwards compatibility bugs are overlooked.
Some text in your website has the wrong color or a box has more padding than it ought to have. Styles may pop up in the wrong places. No big deal, that happens all the time. But some times it can get hairy. Especially with unstructured CSS. So where to start debugging?