Fixed width div on left, fill remaining width div on right

Try removing the float:left and width:100% from .header-right — the right div then behaves as requested.

.header {
  float: left;
  background: #efefef;
  background-repeat: no-repeat;
  width: 240px;
  height: 100px;
}

.header-right {
  overflow: hidden; 
  background-color: #000;
  height: 100px;
}

<div class="header"></div>
<div class="header-right"></div>

Fixed width div on left, fill remaining width div on right

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