Edit in JSFiddle

previousTests();

const children = [
  'width: 61px; height: 32px',
  'width: 123px; height: 86px',
].map(style => h('.child', { style }));
const parent = h('.parent', children);
const dom = h('div', { style: 'width: 513px; height: 324px' }, [parent]);

test((
  'If we have two `.child` elements, they are stacked one over the other'
), { dom, styles }, (is) => {
  is.equal(
    positionOf(children[0]).bottom,
    positionOf(children[1]).top
  );
  is.end();
});

test((
  'Both `.child`ren are centered horizontally'
), { dom, styles }, (is) => {
  children.forEach(child => {
    is.equal(
      positionOf(parent).right - positionOf(child).right,
      positionOf(child).left - positionOf(parent).left
    );
  });
  is.end();
});

test((
  'The distance above the first `.child` equals the distance ' +
  'below the second one'
), { dom, styles }, (is) => {
  is.equal(
    positionOf(parent).bottom - positionOf(children[1]).bottom,
    positionOf(children[0]).top - positionOf(parent).top
  );
  is.end();
});