var rows = document.getElementsByTagName( 'tr' );

for( var i = 0; i < rows.length; i++ ) {
  rows[i].onmouseover = function() {
    this.className += ' hilite';
  }
  
  rows[i].onmouseout = function() {
    this.className = this.className.replace( 'hilite', '' );
  }
}