HTML shows the downsides to the robustness principle. Any lenience in interpreting the spec will lead to a de-facto standard which in turn will make it harder t

HTML shows the downsides to the robustness principle. Any lenience in interpreting the spec will lead to a de-facto standard which in turn will make it harder to create new implementations: It is not enough to follow the spec, you also have to reverse-engineer existing implementations. Only solution is to leave no room for undefined behavior in the spec.