I can't mess it when I edit DOM and browser restores it as it was.
I may have <ul> in <p> (we had it in 1978), I may have <a> in <script> (and it works like comment), I may have <pre>\n and don't worry that it disappear each time I save document. I may have nested <script type="foo"> tags [1].
DOM supports it. XHTML supports it. HTML breaks my content on save-load.
Typically what breaks is styling. It might not be pretty, but it may still be functional.