I am new to css3. Just wanted to know how to center align a div, it should work on web browser, web-kit. I don't have the exact size of the page. I tried 
div.inputs { 
   margin-left:auto;
   margin-right:auto;
}
But this never works for me.
Depending on your HTML you could use display: flex to achieve an easy completely centred element with a small amount of CSS and no additional HTML elements
.container{
    display: flex;
    align-items: center;
    justify-content: center;
}
 
    
    The best way is to define the width, and then set margin to auto & 0:
div.inputs{ margin: 0 auto; width 100%}
 
    
    If you don't know the size of your element, you can use the display: inline-block; with a parent having text-align: center; or you can use a display: table; margin-left: auto; margin-right: auto;
 
    
    Stable Solution (supports flexible height easily):
A more stable code could be look like this which works for vertically & horizontally center a fixed-width, flexible height content:
.outer {
  display: table;
  position: absolute;
  height: 100%;
  width: 100%;
}
.middle {
  display: table-cell;
  vertical-align: middle;
}
.inner {
  margin-left: auto;
  margin-right: auto;
  width: 100px
  /*whatever width you want*/
  ;
}<div class="outer">
  <div class="middle">
    <div class="inner">
      <h1>The Content</h1>
      <p>The inner content :)</p>
    </div>
  </div>
</div>Quick and dirty:
According to your comment on another answer you want it to be centered horizontally and vertically or only horizontally. You can do this by using position absolute:
.centerDiv {
    width:270px;
    height:150px;
    position:absolute;
    left:50%;
    top:50%;
    margin:-75px 0 0 -135px;
    
    background-color: green;
}<div class="centerDiv">Center Me :)</div>If you want it to be centered only horizontally simply use:
.centerDiv {
    width: 270px;
    margin: 0 auto;
}
 
    
    See my answer on this Link here I used just div and span tag to center a div
 
    
    