Validation ≠ Standards All of these parsing 'errors' have clear and straightforward ways to handle them — to the point where the browser can handle an unclosed much faster than it can download and parse ' '. What's worse is that making a parse tree out of markup was never a problem! Consistent CSS layout has always had problems in all rendering engines, but parsing has never been problematic in the slightest. Microso…
However browsers also have graceful handling of invalid and ambigous syntax, and that is a much more controversial issue, because the resolutions have been browser specific and undocumented. This is obviously bad for interoperability.
Draconian error handling is the simplest possible solution to this problem, but HTML5 has a much better (but also much more complex) solution which is to openly specify rules for handling any conceivable syntax error. Microsoft has not peen part of this effort AFAIK.