html - autocomplete attribute is not passing XHTML 1.0 Transitional validation, why?

I'm trying to cleanup my xhtml validation -- I'm running my pages through the W3C validator. For some puzzling reason it's not passing on input fields with the autocomplete="off" attribute:

            onblur="if(this.value=='')this.value=this.defaultValue;" class="searchbox" value="Search" />

I'm using this doctype:

And this is the validation error:

Line 410, Column 81: there is no attribute "autocomplete"

…li_search_1" type="text" autocomplete="off" onfocus="if(this.defaultValue==thi…

I thought this was okay with the W3C -- but, maybe it's still in "submission" phase?
http://www.w3.org/Submission/web-forms2/#autocomplete

Thoughts?

Answer

The Web forms specification has nothing to do with HTML 4 / XHTML. Sadly, autocomplete will not pass validation.

I think the only way to achieve valid HTML 4 /XHTML with autocomplete turned off is adding the attribute on page load using JavaScript. Sucks, I know - but I think it's the only way.