I remember the era (roughly 1998 to 2004) when IE was completely dominant. It sucked if you used an alternative web browser or a non-Windows operating system, which I believe was the point of Microsoft's behavior: protect and maintain the dominance of Windows and the Microsoft ecosystem in general at all costs. Many websites were written with just Internet Explorer for Windows in mind. If you used a different web browser, the web page often didn't render as intended. Even using Internet Explorer for Macintosh wasn't a workaround. Even worse were sites that used ActiveX controls (https://en.wikipedia.org/wiki/ActiveX). While there are ActiveX controls that are not Windows-dependent (ActiveX did run on some non-Windows platforms), in practice many of these controls were Windows-dependent. I remember back in 2005 not being able to use my Linux desktop to fill out an online college financial aid application because the site demanded I use Internet Explorer; I had to use a Windows machine to fill out the application.

Based on what I remember, there were a few factors that eroded Internet Explorer's dominance starting around 2005:

1. Greater awareness of the security problems Windows had during the mid-2000s, especially surrounding Internet Explorer and ActiveX. This encouraged web users to use alternative web browsers, and this encouraged web developers to drop ActiveX for other technologies.

2. The release of Mozilla Firefox in 2004. The browser had an excellent reputation for its speed and its support for the latest web standards (something I'll get to later), and it had tabbed browsing, which Internet Explorer lacked at the time.

3. The Web kept evolving while Internet Explorer's development essentially halted after IE 6 was released. There were new standards that web developers wanted to take advantage of. Firefox took advantage of them, but because IE's development essentially halted, IE 6 didn't take advantage of them. By the time Microsoft finally started work on IE 7 (it was released in 2006), Firefox already received significant mindshare, especially among tech-savvy users.

The latter half of the 2000s and the early years of the 2010s were a golden age for the Web. There was competition in the browser market: Firefox kept advancing, Opera was still popular, IE became competitive again, Safari was a nice browser for the Mac (and even had a Windows port at some point, though I don't know if it was ever popular), and Google released a nice, fast browser named Chrome that started gaining momentum. Web developers generally respected web standards and didn't engage in "Best viewed in X browser" shenanigans like during the Bad Old Days. It was a wonderful time.

Of course, over the 2010s, Chrome became the dominant web browser, Firefox lost momentum, Internet Explorer was replaced by a Chromium fork called Edge, and unfortunately we're starting to see "Best viewed in Chrome" websites and Google having outsized influence over Web standards. There's a new 800 pound gorilla in the jungle.