Output tag in html 5 requires a explicit closing tag, rather than the actual html4 standards.
pass --> <output onforminput="value=slider1.value"> </output>
fails --> <output onforminput="value=slider1.value" />
The HTML5: The Markup Language draft already says that output must have an end tag:
What change are you asking for? And did you intend to file this against the HTML5: The Markup Language or did you mean to file it against the HTML5 spec? (They are two different documents.)