I just spent 2 hours in HTML/CSS/Browser-Compatility-Layouting-Hell again. Worked on a small new website which needed to work on Opera, various versions of FireFox, Safari, InternetExplorer and - although it is nearly dead - IE6 as well. Basically this is quite easy to achieve if you just take a fixed layout and a few images, but doing this with a variable sized layout and multiple files of complicated CSS ensures a lot of fun.
And doing this without special browser version checking really IS hell. Especially if you really need IE6 compatibility, there are some countries
which heavily rely on this browser.
Just thought of never doing web development work again... On the other hand, I promised myself this several times before already. :)