IE6? Meh. You guys have got it easy! You've never had to make CSS layout work in Netscape 4 (without crashing the entire browser)? You've never had to write for appliance browsers that don't support tables? You've never had to write for IE Mobile?
there is no support for JavaScript-assigned event handlers; you can only write an event handler through setting “onclick="somestring"” in innerHTML;
most basic DOM Level 1 properties (eg. nodeName, nodeType, nodeValue, firstChild, lastChild, nextSibling, previousSibling, data, value, HTMLElement.getElementsByTagName, all HTMLTableElement members, most CSSStyleDeclaration members) simply do not exist;
most CSS layout properties do not work; many simply CSS properties like ‘width’ don't work on some elements such as form fields;
setting many other CSS properties on elements like tables and form fields causes an instant browser hang, which, since Windows Mobile has no built-in task manager, means you have to soft-reset the device;
oh, and putting anything but text inside a <button> is insta-crash too;
huge chunks of basic JavaScript methods and “DOM Level 0” methods going back as far as Netscape 2 (!) are missing.
And this is the most up-to-date release of Microsoft's flagship Windows Mobile browser in 2009.
Sure, it supports XMLHttpRequest, but writing AJAX code on a browser whose CSS and script support is less than IE3 (!) is bizarrely schizophrenic, like you're working with a weird amalgam of 21st-century and 19th-century technologies.
I wouldn't recommend it.