HTML

<div class="parent">
    <div class="section yellow" contenteditable>
    Min: 100px / Max: 25%
    </div>
    <div class="section blue" contenteditable>
      This element takes the second grid position (1fr), meaning
      it takes up the rest of the remaining space.
    </div>
  </div>

CSS

        .parent {
  display: grid;
  grid-template-columns: minmax(100px, 25%) 1fr;
}