Among other things, security became a concern. Being lenient is all well and good when the consequences are mild. When the consequences of misinterpreting or i

Among other things, security became a concern. Being lenient is all well and good when the consequences are mild. When the consequences of misinterpreting or interpreting differently to a second implementation becomes costly, such as a security exploit, then the Robustness Principle becomes less obviously a win. It's important to understand that every implementation will try to fix-up formatting problems in their own way unique to their particular implementation. From that you get various desync or reinterpretation attacks (eg. HTTP request smuggling).