.header {
     border: 2px solid #000;
}
 
.wrapper {
     float: left;
     width: 100%;
     margin-right: -304px;
}
 
.content {
     border: 0px solid #000;
     margin-right: 300px;
}
 
.sidebar {
     float: right;
     width: 300px;
     border: 0px solid #000;
}
 
.footer {
     clear: both; 
     border: 2px solid #000;
}
 
@media (max-width: 1000px) {
     .wrapper, .content, .sidebar {
          float: none;
          margin: 0;
          width: auto;
     }
}