IE5 is the last version of IE to support Windows 3.1x, NT 3.51, 95, and NT 4 SP3-SP6. It also supported Solaris, HP-UX, and mac OS X. It had support for HTML 4.01, CSS1, CSS2, MHTML, XSLT, XML, XHR, HTA, and DHTML (Dynamic HTML).
DHTML uses HTML, JS, and CSS to create dynamic pages. DHTML was supported to some extent by IE4+ and Netscape Navigator 4.0+, as well as Firefox 1.0+ and Opera 7+. However, at the time of IE4 and Netscape Navigator 4, the DOM was not completely standardized, and so differences could be seen between IE and Netscape Navigator (although Netscape did try to promote "cross-browser DHTML"). This is commonly called DOM Level 0 (aka. Legacy DOM).
DOM Level 1, released by the W3C in 1998, is the first effort in standardizing the DOM, taking inspiration from DHTML but not implementing all of DHTML. However, it will take a while before Microsoft and Netscape focus on DOM Level 1 support over their own proprietary DOMs in their own DHTML systems. DOM Level 1 gets partial support only in IE6+, which is not available for Windows NT 4 SP6 and below. It also gets support in Netscape Navigator 6 and Mozilla Firefox 1.0.
DOM Level 2 was released by the W3C in 2000 and includes an events model that was absent in DOM Level 1. It also has support for XML namespaces and CSS.
DOM Level 3 was released in 2004 and supports XPath, keyboard event handling, and an interface for serializing documents as XML.