This is the most common place where people run into issues. They create something in one browser, and then find that the way it's rendered in another simply isn't the same because there are different unspoken assumptions at work about how the standard is to be implemented.
IE's implementation of Web standards includes a whole slew of ad hoc additions to the HTML tag library that don't show up elsewhere. Most of these seem to have been on the wane, but every now and then you run across a site where they still haven't been eliminated.
A simple example: The bgproperties attribute for the body tag allowed you to keep a background image on a page stationary while someone scrolls through the page. This particular method doesn't work in other browsers other than IE, but there is a CSS style attribute -- background: fixed -- that accomplishes the same thing on both IE and Mozilla.
Another proprietary tag I've seen used in IE, but not widely (thank goodness), and that can and should be done away with, is the CSS font / image filter system. This employs the filter: statement in a CSS style declaration to add various kinds of post-processing to images and fonts directly on a Web page. There are some examples on David J. Hark's Web site; view this page in IE and then in Firefox to see the differences. This is the sort of thing that can be dropped into the same trash can as the blink tag, if only in the name of good taste.