[html] How to make a div fill a remaining horizontal space?

Since this is a rather popular question, I'm inclined to share a nice solution using BFC.
Codepen sample of the following here.

.left {
  float: left;
  width: 100px;
}
.right {
  overflow: auto;
}

In this case, overflow: auto triggers context behavior and makes the right element expand only to the available remaining width and it will naturally expand to full width if .left disappears. A highly useful and clean trick for many UI layouts, but perhaps hard to understand the "why it works" at first.

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