alt Hacker NewsI generally agree, but I don't think XML is the best example. Getting HTML out of XML is considered to have been the right move isn't it? I was pro-XHTML2 at the time but in retrospect, have we suffered much for not sending webpage validation errors to end users?
Once people have gotten used to not having to conform, forcing them to conform is an uphill battle. Doubly so when, as happened with Microsoft and IE, the vendors would like to encourage vendor lock-in. The only time to reasonably do it is at the start.
That said, we are paying a huge complexity cost due to our efforts to allow nonconforming pages. This complexity is widely abused by malicious actors. See, for instance, https://cheatsheetseries.owasp.org/cheatsheets/XSS_Filter_Ev... for ways in which attackers try to bypass security filters. A lot of it is only possible because of this unnecessary complexity.