nav-sample.css

nav ul {
  width: 100%;
  display: table;

  aside & { // in this case, our submenu markup is in the aside
    display: block;
    width: 150px; }
  
  li {
    display: table-cell;
    text-align: center;

    aside & {
      clear: left;
      display: block;}
    
    a {
      display: block;

      aside & {
        text-align: left;}
    }
  }
}

/* Output*/
nav ul { width: 100%; display: table; }
aside nav ul { display: block; width: 150px; }
nav ul li { display: table-cell; text-align: center; }
aside nav ul li { clear: left; display: block; }
nav ul li a { display: block; }
aside nav ul li a { text-align: left; }