style.css

.wrapper {
  display: flex;
  flex-flow: row wrap;
  font-weight: bold;
  text-align: center;
}

.wrapper > * {
  padding: 10px;
  flex-basis: 100%;
}

.header {
  background: tomato;
}

.footer {
  background: lightgreen;
}

.main {
  text-align: left;
  background: deepskyblue;
}

.aside-1 {
  background: gold;
}

.aside-2 {
  background: hotpink;
}

@media all and (min-width: 400px) {
  .aside {
    flex: 1 0; /* notation abrégée */
    /*
    flex-basis: 0;
    flex-grow: 1;
    */
  }
}

@media all and (min-width: 600px) {
  .main {
    flex: 3 0;
  }
  .aside-1 {
    order: 1;
  }
  .main {
    order: 2;
  }
  .aside-2 {
    order: 3;
  }
  .footer {
    order: 4;
  }
}

body {
  padding: 2em;
}