Setting a length (height or width) for one element minus the variable length of another, i.e. calc(x – y), where y is unknown

.wrapper {
  display: table;
  width: 100%;
  margin: 15px 0;
}

.horizontal.wrapper > div {
  display: table-cell;
  white-space: nowrap; /* Prevent line wrapping */
  border: 1px solid;
}
.left { width: 100px } /* Minimum width of 100px */
.center { width: 0; }  /* Width given by contents */

.vertical.wrapper { height: 200px; }
.vertical.wrapper > div {
  display: table-row;
}
.vertical.wrapper > div > span {
  display: table-cell;
  border: 1px solid;
}
.top    { height: 100px; } /* Minimum heigth of 100px */
.middle { height: 0; }     /* Height given by content */
.bottom { height: 100%; }  /* As tall as possible */

<div class="horizontal wrapper">
  <div class="left">100px wide</div>
  <div class="center">Auto width, given by contents</div>
  <div class="right">Remaining space</div>
</div>
<div class="vertical wrapper">
  <div class="top"><span>100px tall</span></div>
  <div class="middle"><span>Auto height, given by contents</span></div>
  <div class="bottom"><span>Remaining space</span></div>
</div>

#wrapper, .right { overflow: hidden; } /* Establish BFC */
#wrapper > div { border: 1px solid; }
.left, .middle { float: left; }
.left { width: 100px }

<div id="wrapper">
  <div class="left">100px</div>
  <div class="middle">Auto width, given by contents</div>
  <div class="right">Remaining space</div>
</div>