Владислав Поляков

<div class="Grid">
  <div class="Grid-cell">…</div>
  <div class="Grid-cell">…</div>
</div>

.Grid {
  display: flex;
}

.Grid-cell {
  flex: 1;
}

.content-inner {
    width: 300px;
    height:300px;
    background: #FFF;
    box-shadow: 0 1px 2px rgba(0,0,0,0.2);
    position:relative;
}
.content-inner::before {
    content: "";
    display: block;
    border: 11px solid transparent;
    border-right-color: #DDD;
    position: absolute;
    top: 47px;
    left: -22px;
}
.content-inner::after {
    content: "";
    display: block;
    border: 10px solid transparent;
    border-right-color: #FFF;
    position: absolute;
    top: 48px;
    left: -20px;
}

.span12

@media screen and (max-width: 980px) {
   #pagewrap {
      width: 95%;
   }
   #content {
      width: 60%;
      padding: 3% 4%;
   }
   #sidebar {
      width: 30%;
   }
   #sidebar .widget {
      padding: 8% 7%;
      margin-bottom: 10px;
   }
}