test overview:

whatwg HTML5 TR [current living standard] states that the default mime type for type= attribute on <style></style> in HTML5 is "text/css".

results

this text should be green.

This default type feature of the style element for HTML5 is NOT working according to whatwg.org TR.

style type is ""

text color type is ""

As it is, p.style.color is only accessible to js when style attribute is applied directly to element. that is why this code fails in ff.