<div class="tab-head">
<h2 id="tab1" onmouseover="changeTab1()" class="selected">1</h2>
<h2 id="tab2" onmouseover="changeTab2()">2</h2>
<h2 id="tab3" onmouseover="changeTab3()">3</h2>
</div>
<div class="tab-content">
<div id="c1" class="show">content1</div>
<div id="c2">content2</div>
<div id="c3">content3</div>
</div>

h2 {
border-top: solid cornflowerblue 1px;
border-left: solid cornflowerblue 1px;
width: 50px;
height: 25px;
margin: 0;
float: left;
text-align: center;
}
.tab-content {
border: solid cornflowerblue 1px;
width: 152px;
height: 100px;
}
.tab-content div{
display: none;
}
.selected {
background-color: cornflowerblue;
}
.tab-content .show{
display: block;
} 

var tab1 = document.getElementById('tab1'),
tab2 = document.getElementById('tab2'),
tab3 = document.getElementById('tab3'),
c1 = document.getElementById('c1'),
c2 = document.getElementById('c2'),
c3 = document.getElementById('c3');
function changeTab1() {
tab1.className = 'selected';
tab2.className = '';
tab3.className = '';
c1.className = 'show'
c2.className = '';
c3.className = '';
}
function changeTab2() {
tab1.className = '';
tab2.className = 'selected';
tab3.className = '';
c1.className = '';
c2.className = 'show';
c3.className = '';
}
function changeTab3() {
tab1.className = '';
tab2.className = '';
tab3.className = 'selected';
c1.className = ''
c2.className = '';
c3.className = 'show';
} 

<div class="tab-head">
<h2 onmouseover="changeTab(0)" class="selected">1</h2>
<h2 onmouseover="changeTab(1)">2</h2>
<h2 onmouseover="changeTab(2)">3</h2>
</div>
<div class="tab-content">
<div class="show">content1</div>
<div>content2</div>
<div>content3</div>
</div> 

var tabs = document.getElementsByClassName('tab-head')[0].getElementsByTagName('h2'),
contents = document.getElementsByClassName('tab-content')[0].getElementsByTagName('div');
function changeTab(index) {
for(var i = 0, len = tabs.length; i < len; i++) {
if(i === index) {
tabs[i].className = 'selected';
contents[i].className = 'show';
}else{ tabs[i].className = '';
contents[i].className = ''; }
}
}

<div class="tab-head">
<h2 onmouseover="changeTab(this)" class="selected">1</h2>
<h2 onmouseover="changeTab(this)">2</h2>
<h2 onmouseover="changeTab(this)">3</h2>
</div>
<div class="tab-content">
<div class="show">content1</div>
<div>content2</div>
<div>content3</div>
</div> 

var tabs = document.getElementsByClassName('tab-head')[0].getElementsByTagName('h2'),
contents = document.getElementsByClassName('tab-content')[0].getElementsByTagName('div');
function changeTab(tab) {
for(var i = 0, len = tabs.length; i < len; i++) {
if(tabs[i] === tab) {
tabs[i].className = 'selected';
contents[i].className = 'show';
} else {
tabs[i].className = '';
contents[i].className = '';
}
}
} 

<div class="tab-head">
<h2 class="selected">1</h2>
<h2>2</h2>
<h2>3</h2>
</div>
<div class="tab-content">
<div class="show">content1</div>
<div>content2</div>
<div>content3</div>
</div> 

var tabs = document.getElementsByClassName('tab-head')[0].getElementsByTagName('h2'),
contents = document.getElementsByClassName('tab-content')[0].getElementsByTagName('div');
(function changeTab(tab) {
for(var i = 0, len = tabs.length; i < len; i++) {
tabs[i].onmouseover = showTab;
}
})();
function showTab() {
for(var i = 0, len = tabs.length; i < len; i++) {
if(tabs[i] === this) {
tabs[i].className = 'selected';
contents[i].className = 'show';
} else {
tabs[i].className = '';
contents[i].className = '';
}
}
}