Two Divs next to each other, that then stack with responsive change

today this kind of thing can be done by using display:flex;

https://jsfiddle.net/suunyz3e/1435/

html:

  <div class="container flex-direction">
      <div class="div1">
        <span>Div One</span>
      </div>
      <div class="div2">
        <span>Div Two</span>
      </div>
  </div>

css:

.container{
  display:inline-flex;
  flex-wrap:wrap;
  border:1px solid black;
}
.flex-direction{
  flex-direction:row;
}
.div1{
  border-right:1px solid black;
  background-color:#727272;
  width:165px;
  height:132px;
}

.div2{
  background-color:#fff;
  width:314px;
  height:132px;
}

span{
  font-size:16px;
    font-weight:bold;
    display: block;
    line-height: 132px;
    text-align: center;
}

@media screen and (max-width: 500px) {
  .flex-direction{
  flex-direction:column;
  }
.div1{
  width:202px;
  height:131px;
  border-right:none;
  border-bottom:1px solid black;
  }
  .div2{
    width:202px;
    height:107px;
  }
  .div2 span{
    line-height:107px;
  }

}

You can use CSS3 media query for this. Write like this:

CSS

.wrapper { 
  border : 2px solid #000; 
  overflow:hidden;
}

.wrapper div {
   min-height: 200px;
   padding: 10px;
}
#one {
  background-color: gray;
  float:left; 
  margin-right:20px;
  width:140px;
  border-right:2px solid #000;
}
#two { 
  background-color: white;
  overflow:hidden;
  margin:10px;
  border:2px dashed #ccc;
  min-height:170px;
}

@media screen and (max-width: 400px) {
   #one { 
    float: none;
    margin-right:0;
    width:auto;
    border:0;
    border-bottom:2px solid #000;    
  }
}

HTML

<div class="wrapper">
    <div id="one">one</div>
    <div id="two">two</div>
</div>

Check this for more http://jsfiddle.net/cUCvY/1/