Because, sometimes, standards are wrong, purposefully allow implementation-specific choices, or are helplessly vague and everyone can't agree on the interpretation.

However, the right answer is usually to exceed the requirements of the standard in way that best benefits the consumer and meets business goals.

> But why was the non-standard way even an option?

People tend to have their own ideas on how things should be done. They should definitely be heard too. There is nothing wrong with questioning or doing things differently than current standards dictate as long as you have open, honest discussions.

Many of the IE specific stuff dates back to the IE4/5 era, and back then Netscape was not much better.