demo.js

render: function() {
  return (
    <Grid>
      <GridCol width="1-of-2">
        <p>Hello, world!</p>
      </GridCol>
      <GridCol width="1-of-2">
        <p>This is a column!</p>
      </GridCol>
    </Grid>
  );
}

// Output:
// <div class="o-grid">
//   <div class="o-grid__col o-grid__col--1-of-2">
//     <p>Hello, world!</p>
// [etc]