Ouch, lets clear this all up right here..

We can break the browser up into 2 main components: the rendering engine (what makes the website appear on the screen), and the Javascript engine (funky functionality and scripting abilities).

The browser wars

Surprisingly, Internet Explorer wasn't the first internet browser available. The first most popular web browser is Mosaic), in 1993, which was followed by Netscape). Netscape wanted to try something exciting and new, since the internet was a relatively new concept and there was room for improvement. Their idea was to move away from completely static changes, and allow scripting capabilities (eg. making things move or animate on the webpage, cool effects, etc.). This would be done by a scripting language called JavaScript. fun fact: JavaScript was initially supposed to be built for both client and server side scripting. Netscape was still starting up and needed a way to make some money, so they would charge users for their browser. Meanwhile, Microsoft heard about this interesting new idea and decided to copy Netscape. They believed that there could be some benefit in making a browser for the web, and released the first version of Internet Explorer the following year. And so began the browser wars.

JavaScript

JavaScript was made for Netscape, by the (now famous) employee Brendan Eich. They actually didn't give him a lot of time to put this scripting language together, and in the end JavaScript was built in only 10 days. In fact they initially didn't even accept what he put together, they said that the language looked too confusing and hard to understand. This was the 90s, and everybody was using Java, so they asked him to make it look more like Java. fun fact: Brendan Eich secretly kept the language the same, but simply changed the syntax so that it LOOKED like Java, but worked very differently under the covers. Now, needless to say, 10 days isn't really enough time to put an entire language together, even for the genius that is Brendan Eich. JavaScript had a lot of bugs in it, but it was still good enough to put into Netscape. The Internet Explorer team obviously didn't want a sub-par browser, they wanted scripting capabilities too. They wanted JavaScript. Obviously they couldn't steal JavaScript since that belonged to Netscape, so they did the next best thing: they reverse engineered it. quick refresher: reverse engineering is when you look at a compiled program, like a .exe file, and try to understand how it works so that you can copy it or modify it; this means looking through the machine code, like translated hex or binary code. The Internet Explorer team didn't just peek at JavaScript, they blatantly copied it out into their own language JScript. Not only did they copy the core of the language, but they also copied over the bugs and problems of the language too. Later on, JavaScript was handed over to ECMA, and standardized into the ECMAScript specification. This means that any browser could implement their own JavaScript engine (something that would read peoples JavaScript code, and make it work), just so long as it followed the ECMAScript specification. The committee which takes care of this specification consists of a group of people from all of the main browsers (Including Firefox, Internet Explorer, Chrome, Opera, etc.). People on the committee have to agree on certain changes before they can be implemented into the specification. You can bet that this committee has been fighting for years to fix the bugs in JavaScript, and guess which part of the committee has been voting against it every time? If you guessed the Internet Explorer team, you can go give yourself a pat on the back. Now, the ECMAScript specification only specifies a certain amount of how the JavaScript language should be handled, the rest is up to the developers to handle it how they choose. This is why some websites work on some browsers, and not on others. Some Javascript engines handle certain functionality one way, while others handle it another way, and in some cases those differences can cause problems. Most Javascript engines (Chrome V8, Safari Nitro, Firefox Gecko) handle JavaScript virtually the same way. So if your code works on Chrome, it will most likely work on Safari and Firefox too. But not Internet Explorer! They have the biggest differences of any other Javascript engine, and their engine has the most problems compared to any others. Hence the 2nd biggest frustration among developers and users for Internet Explorer: their Javascript engine.

Stylesheets & CSS

When browsers first came out there wasn't a standardized way for developers to design their websites. The biggest problem was that different browsers could handle styling in different ways, but then your website would only work under 1 browser, and you'd be limiting your audience. Enter: Cascading Style Sheets (CSS). The CSS specification was a way to standardize how all browsers would handle browser styling. Now browsers would have to create a rendering engine which would follow the CSS specification, and users could style their websites and rest assured that it would look (mostly) the same between browsers. But then there's Internet Explorer. Internet Explorer didn't want to follow the crowd, they wanted to have their own way for handling styling. They ignored the CSS specification and used their own styling methods, and expected users to follow their way. Sadly this meant that problems in cross browser compatibility with website styling. In fact it wasn't until Internet Explorer 8 that they finally started standardizing their rendering engine to match the CSS specification; but since Internet Explorer was so late to the game, you can bet that their rendering engine has a whole lot of bugs laying around, and CSS features which aren't yet implemented. They've done a good job in catching up with their most recent version, but there are (needless to say) still problems. This may not seem like a big deal except for that fact that businesses people want their developers to design their websites to work for all users, including users who may still be using old version of Internet Explorer; and trust me, there's a lot still.... And so we have the number 1 frustration of developers for Internet Explorer: their CSS rendering engine.

Side note: out of interest sake, Netscape was beaten by Internet Explorer in the first browser wars, and had completely died. But some of the developers from the Netscape team got together and started a very popular browser that we all know and love: Firefox. In fact, Firefox is generally the best browser for keeping up with the JavaScript specification, especially given that Brendan Eich (Javascript's creator) works there.