A flaw can become entrenched as a de facto standard. Any implementation of the protocol is required to replicate the aberrant behavior, or it is not interoperable. This is both a consequence of applying the robustness principle, and a product of a natural reluctance to avoid fatal error conditions. Ensuring interoperability in this environment is often referred to as aiming to be "bug for bug compatible".

In short, while it’s technically possible to make the part before @ case sensitive, most popular email servers do not allow that. 

Either way, at some point almost everyone started treating addresses as case-insensitive.

The de facto standard is to treat local mailboxes as case insensitive, but the official standard says case matters (though even the official standard mentions the de facto standard).

supports standard Markdown (with some minor modifications) and various extensions that have been made popular by the PHP Markdown Extra package and Maruku

We believe that Kubernetes is the defacto standard for composing Pods and for orchestrating containers, making Kubernetes YAML a defacto standard file format.

AnyCable allows you to use any WebSocket server (written in any language) as a replacement for your Ruby server (such as Faye, Action Cable, etc).

jq uses the Oniguruma regular expression library, as do php, ruby, TextMate, Sublime Text, etc, so the description here will focus on jq specifics.

Again, this is all opinion-based, and due to the sheer number of developers who rely on this technology as their bread and butter, sub-communities and religiousness forms around patterns, anti-patterns, practices, de-facto standards, micro-packages, polyfills, frameworks, build-tools, etc.

All browers handle 302 incorrectly. Chrome 30, IE10. It became the de facto incorrect implementation; that cannot be changed because so many web-sites issue mistakenly issue 302. In fact ASP.net MVC incorrectly issues 302, depending on the fact that browsers handle it incorrectly.