CSS fill remaining width

Answer 1

Use calc!

https://jsbin.com/wehixalome/edit?html,css,output

HTML:

  100 px wide!
  

    Fills width!
  

CSS:

.left {
  display: inline-block;
  width: 100px;

  background: red;
  color: white;
}
.right {
  display: inline-block;
  width: calc(100% - 100px);

  background: blue;
  color: white;
}

Update: As an alternative to not having a space between the divs you can set font-size: 0 on the outer element.