Along similar lines, there was a security vulnerability in Netscape Navigator because its HTML parser treated Single Left-Pointing Angle Quotation Mark as if it

Along similar lines, there was a security vulnerability in Netscape Navigator because its HTML parser treated Single Left-Pointing Angle Quotation Mark as if it were Less-Than Sign and Single Right-Pointing Angle Quotation Mark as if it were Greater-Than Sign, presumably because they looked the same and the developers somehow thought that it would be helpful. Of course no markup sanitiser behaved the same and it became a really easy XSS vector because an attacker could use those to give markup to Netscape that would pass through any sanitiser unmodified. There’s loads of examples of Postel’s Law causing security vulnerabilities like this.