5px extra margin getting added to the bottom of each div

Question

I am trying to design a simple header to a page in css. I planned to stack two divs on top of each other. The top one has many tabs and the bottom one is a plain solid single image div. But when rendering i see that an extra 5px is getting added to the the heights of both these divs. So i am not able to place the bottom on exactly on top of the other one.

There is a 5px bottom margin automatically. I tried negative margins, reset the global margins and paddings to zero. Still no use.

Heres the code.

<div class ="main_nav">
 <div class="first_tab">
 <img src ="images/startup/tab1_brown.png" height="25" width="90" alt="Temp" /> 
 </div>
 <div class = "ind_tab">
 <img src ="images/startup/tab1_orange.png" height="25" width="90" alt="Temp"/>
 </div>
 <div class = "ind_tab">
 <img src ="images/startup/tab1_brown.png" height="25" width="90" alt="Temp" />
 </div>
</div>
<div class="lock">
 <img src ="images/startup/divbg_new.png" alt="Temp" />
</div>

CSS:

 *{ margin:0; padding:0; }
ul.master_navigation
{
    font-size: 125%;
    font-weight: bold;
    text-align: center;
    list-style: none;
    margin: 0.5em 0;
    padding: 0;
}

ul.master_navigation li
{
    display: inline-block;
    padding: 0 1%;
}

a
{
    color: #08f;
    font-weight: bold;
    text-decoration: none;
}

a:visited
{
    color: #88f;
}

a:hover
{
    color: #f00;
}

p
{

    text-align: justify;
}


p.cent
{
    text-align: left;
}

div.header
{    
    height: 200;
}

div.main_nav
{ 
    display: inline-block;
    height: 25;

    width: 900;
    padding: 0;
    margin: 0;
    vertical-align: bottom;

}

div.first_tab
{    
    height: 25;
    float: left;


}
div.ind_tab
{    

    height: 25;
    float: left;
    margin-left: -10px;
    z-index: -5;

}

div.lock
{
    margin-top: -100;
    height: 91;
    width: 900;
    padding: 0;
    margin: -5;

}

body {

    width:900px;
    max-width: 100%;
    margin: auto;
    padding: 0;
    background-image:url(images/startup/bg_2.gif);
    background-repeat:repeat-x;
}

.pad 
{
    padding: 0;
    margin: 0;

}

Here's the link to the page

http://net4.ccs.neu.edu/home/pradep/

Ive been spending too much time on this. Please help.

Answer 1

Ege's answer was very useful for me. I have spent hours to find the reason of a bottom padding of some images in div's. It was the line-height. Set it to 0px on the interesting areas:

.divclass {
    line-height: 0px; /* crucial for bottom padding 0 !!! */
}