The strict vs. lenient in this case also comes down to extensibility. HTML for example is notoriously forgiving and HTML parsers are quite complex because of i

The strict vs. lenient in this case also comes down to extensibility. HTML for example is notoriously forgiving and HTML parsers are quite complex because of it but it allowed HTML to be extended and improved upon while maintaining backwards compatibility. If the first web browser was extremely strict on the tags and structure it supported then web could not have evolved that way that it did.