I came to hate IE with every fiber of my being, but let's call a spade a spade here: IE basically invented modern CSS. Netscape 3/4 and IE 3/4 had very different views on how web pages should be dynamic and how they should be styled.
IE championed CSS and the DOM. Netscape was fixated on bgcolor, font tags, etc. Netscape introduced the tag. IE just made every tag a potential layer. Remember AJAX? Loading content from a script and injecting it into a page? That was 100% IE.
The issue wasn't that IE 5, 5.5, and 6 were bad when they came out. They weren't. Not one bit. They were absolutely the cutting edge. Then Netscape died, Microsoft had no more competition in the field, and so they basically just stopped. For years.
Mozilla (and shortly after, Firefox) prompted an eventual response, which was IE7: I minor update to an otherwise abandoned codebase. Once it was clear folks were seriously looking at dumping IE, IE8 followed shortly after. The CSS Acid Test showed how far IE had fallen behind, so IE9 and IE10 moved to stop the bleeding. IE 11 was the final release before Microsoft finally dropped the legacy IE baggage, aka the Trident engine. By that time Chrome had stormed the field, and Edge's renderer just couldn't keep up relative to the amount of money Microsoft was willing to pay. So they dumped their own engine and moved to Blink. (If you can't beat em…)
But let's be clear, between 1998 and 2003, IE was at the forefront. Its CSS support wasn't broken. It was the best and only. Its box model wasn't broken. There were no other box models to compare it to. You didn't need to give it separate CSS, because it was the only one around really to parse it. It didn't have non-standard AJAX support. It WAS the standard since no XMLHttpRequest object existed at the time.
You could write VBScript on a web page instead of JavaScript and have a >90% chance that client could run it. Flash didn't have to be bundled or installed separately, ActiveX just took care of it in the middle of your browsing session.
It was the Bronze Age of the web. IE was the one that moved us beyond stone tools. Later supplanted by Firefox and Safari's Iron Age, but still an important and arguably necessary step toward the modern web we know today, including the box model.