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The validator incorrectly accepts a self close for the script tag as valid html syntax. However, the html specs require a </script> follow a <script ... > tag ie. <script ... /> is treated the same way as <script ... ></script> All of the browsers will gobble up the entire rest of html page looking for the </script> close tag, and therefore display nothing. I was cleaning up a web page changing all of the <br> tags to <br />, etc, and I absentmindedly collapsed a <script ... ></script> pair down to <script ... />. The validator accepted the self close syntax, but the page no longer renders.
It is not a validation error to use <script ... /> in XHTML, but as you noted, doing so does have compatibility issues with browsers.