Originally I had this in a media query and everything was ok.

Code CSS:
@media (max-width: 870px){
    #menu {
        left: 0;
        padding-left: 0;
        position: relative;
        display: block;

Then I changed #menu to nav ( changed all coresponding code in my .html and .css file )

and my page breaks. Changed it back and it works.

Do I div#menu or nav#menu. Do media queries not understand new HTML5 elements?