HTML5 is strict too. But it is strict in a pragmatic way, which makes it much nicer to write and read. Instead of forcing authors to write verbose code to an exacting spec without errors, they allowed authors to take shortcuts and make errors. But crucially, then they rigorously specified how those shortcuts and errors must be handled in all cases.
It doesn't matter if all your angle brackets are in the right place or all your attributes are quoted. What matters is that all implementations do the same thing given the same input. You can be strict and achieve interoperability without punitive error handling.
Wasn't this the goal of xhtml?