<style>
body {
  margin: 0;
}

.row {
  clear: both;
}

.row div {
  float: left;
}

.red {
  background-color: red;
}
</style>

<div class="row">
  <div class="red" style="width: 200px">a</div>
  <div style="width: 200px">a</div>
</div>

<div class="row">
  <div class="red" style="width: 200px">a</div>
  <div style="width: 200px">a</div>
</div>

<div class="row">
  <div class="red" style="width: 100px">a</div>
  <div style="width: 300px">a</div>
</div>

<div class="row">
  <div class="red" style="width: 100px">a</div>
  <div style="width: 300px">a</div>
</div>