CSS selector for disabled input type=”submit”

Question

Does that work in IE6?

No, IE6 does not support attribute selectors at all, cf. CSS Compatibility and Internet Explorer.

You might find How to workaround: IE6 does not support CSS “attribute” selectors worth the read.

EDIT
If you are to ignore IE6, you could do (CSS2.1):

input[type=submit][disabled=disabled],
button[disabled=disabled] {
    ...
}

CSS3 (IE9+):

input[type=submit]:disabled,
button:disabled {
    ...
}

You can substitute [disabled=disabled] (attribute value) with [disabled] (attribute presence).