I'm on Windows 8.1 with Mathematica 9.
Export["test.html",EvaluationNotebook[],"MathOutput"\[RightArrow]"GIF","CSS"->None]
Export["test.html",EvaluationNotebook[],"CSS"->None,"MathOutput"\[RightArrow]"GIF"]
Export::argt: Export called with 4 arguments; 2 or 3 arguments are expected. >>
Export::noelem: MathOutput[RightArrow]GIF is not a valid set of export elements for the HTML format. >>
I think picture is clear to show the problem.

"CSS" -> None, "MathOutput" \[RightArrow] "GIF" are options? right? and order sensitive?
"CSS" -> None seems to be an argument? of course it's an option in the help page of HTML,

Export::noelem: MathOutput\[RightArrow]GIF is not a valid set of export elements for the HTML format.This indicates you can't export aManipulateas a GIF. – m_goldberg Aug 22 '13 at 15:48Export[ FileNameJoin[{$HomeDirectory, "Desktop", test.html"}], Plot[Sin[t], {t, -6, 6}]]produces a bad HTML file. – m_goldberg Aug 22 '13 at 18:03