I did notice the difference and the structure is used is similar to the real code of moodle 1.6.
As I understand an html item with class="correct" wil appear as green either as a direct descendant of que.multichoice in this case it will be que.multichoice or as an element inside another element itself a direct descendant of que.multichoice .
The real problem is that structure is official Moodle code and it is working consistenly in different browser.
There is a similar problem than clicking to the attachment file test2.html with EE or FF the bold text is not at the same place.
What it is important is that if the CSS coding is not done correctly , the display can be (and it is in some part Moodle) because the browser are different.
The Moodle CSS coding should correct this and give consistant user interface.
I think that this problem has not been set correctly in Moodle and that this is a more general problem that the question display.
Somebody somewhere says "Houston, we have a problem!"
This will not cause error to the student results but they will surely be anxious that this is not the case.
I am also very anxious if somebody code somewhere in the styles_layout.css that the default class ".invisiblefieldset " is ... .
I expect the all local uses of class="invisiblefieldset" protected by another definition like .que.multichoice. .invisiblefieldset will be identically recognized local by all current browsers.
Actually this is not the case and the results vary.
This is why screenshots-5 is GREEN and screenshots-6 is NOT
I will set tonight more clearer examples.
As a newcomer in CSS, I don't know how to code (almost) universal CSS, I just notice that Quick HTML seems to have part of the solution (at least a greater part than the actual Moodle CSS.)