div.navigation{
width: 800px;
height: 40px;
background: red;
margin: 4px auto 0;
top: 400px;
left: 0px;
position: fixed;
}

//記錄導航條原來在頁面上的位置
var naviga_offsetTop = 0;
var naviga_offsetLeft = 0;

//IE7不識別getElementsByClassName，為了兼容自定義一個
function my_getElementsByClassName(class_name) {
var el = [];
//獲取所有元素
_el = document.getElementsByTagName('*');
//通過className刷選
for (var i=0; i<_el.length; i++ ) {
if (_el[i].className == class_name ) {
el[el.length] = _el[i];
}
}
return el;
}

//導航條，懸停在頂部
function naviga_stay_top(){
var a_navigation_bar = [];
if(document.getElementsByClassName){//Chrome, FF
a_navigation_bar = document.getElementsByClassName("navigation");
} else {//IE
a_navigation_bar = my_getElementsByClassName("navigation");
}
var scrollTop = document.body.scrollTop || document.documentElement.scrollTop;

document.title = scrollTop;
//如果向下滾動的距離大于原來導航欄離頂部的距離
//直接將導航欄固定到可視區頂部
if( scrollTop > naviga_offsetTop ){
a_navigation_bar[0].style.top = 0 + "px";
} else {
//如果向下滾動的距離小原來導航欄離頂部的距離，則重新計算導航欄的位置
a_navigation_bar[0].style.top = (naviga_offsetTop - scrollTop) + "px";
}
}

//給導航條上四個tab，加上點擊事件。
window.onload=function(){
var a_tabs = [];
if( document.getElementsByClassName ){//Chrome, FF
a_tabs = document.getElementsByClassName("tab");
}else{ //IE
a_tabs = my_getElementsByClassName("tab");
}

var a_contents = [];
if( document.getElementsByClassName ){//Chrome, FF
a_contents = document.getElementsByClassName("content");
}else{//IE
a_contents = my_getElementsByClassName("content");
}

//獲取offsetLeft，即導航欄離左邊框的距離
var a_main_div = [];
if( document.getElementsByClassName ){//Chrome, FF
a_main_div = document.getElementsByClassName("main");
}else{ //IE
a_main_div = my_getElementsByClassName("main");
}
naviga_offsetLeft = a_main_div[0].offsetLeft;

a_tabs[0].onclick=function(){
window.scrollTo(0, a_contents[2].offsetTop);
}
a_tabs[1].onclick=function(){
window.scrollTo(0, a_contents[3].offsetTop);
}
a_tabs[2].onclick=function(){
window.scrollTo(0, a_contents[4].offsetTop);
}
a_tabs[3].onclick=function(){
window.scrollTo(0, a_contents[5].offsetTop);
}

//獲取頁面上，導航條到頂部的位置
var a_navigation_bar = [];
if(document.getElementsByClassName){//Chrome, FF
a_navigation_bar = document.getElementsByClassName("navigation");
} else {//IE
a_navigation_bar = my_getElementsByClassName("navigation");
}
//獲取offsetTop
naviga_offsetTop = a_navigation_bar[0].offsetTop;
a_navigation_bar[0].style.left = naviga_offsetLeft + "px";

//給滾動條以及鼠標加上滾動事件
if( window.attachEvent) //IE
{
window.attachEvent("onmousewheel", naviga_stay_top);
window.attachEvent("onscroll", naviga_stay_top);

document.attachEvent("onmousewheel", naviga_stay_top);
document.attachEvent("onscroll", naviga_stay_top);
} else {//Chrome ,FF
window.addEventListener("mousewheel", naviga_stay_top,false);
window.addEventListener("scroll", naviga_stay_top,false);

document.addEventListener("mousewheel", naviga_stay_top,false);
document.addEventListener("scroll", naviga_stay_top,false);
}
}