React

HTML

<div id="app"></div>

CSS

body {
  background: #20262E;
  padding: 20px;
  font-family: Helvetica;
}

#app {
  background: #fff;
  border-radius: 4px;
  padding: 20px;
  transition: all 0.2s;
}

li {
  margin: 8px 0;
}

h2 {
  font-weight: bold;
  margin-bottom: 15px;
}

.done {
  color: rgba(0, 0, 0, 0.3);
  text-decoration: line-through;
}

input {
  margin-right: 5px;
}

React

const Rect = ({width, height, round, strokeWidth}) => {
    // overhang over given width and height that we get due to stroke width
    const s = strokeWidth / 2;
    // how many pixels do we need to cut from vertical and horizontal parts
    // due to rounded corners and stroke width
    const over = 2 * round + strokeWidth;
    // lengths of straight lines
    const w = width - over;
    const h = height - over;
    const d = `
        M${round + s},${s}
        h${w}
        a${round},${round} 0 0 1 ${round},${round}
        v${h}
        a${round},${round} 0 0 1 -${round},${round}
        h-${w}
        a${round},${round} 0 0 1 -${round},-${round}
        v-${h}
        a${round},${round} 0 0 1 ${round},-${round}
        z
    `;
    return (
        <svg width={width} height={height}>
            <path d={d} fill="none" stroke="black" strokeWidth={strokeWidth} />
        </svg>
    );
};

ReactDOM.render(
    <Rect width={64} height={32} strokeWidth={2} round={4} />,
    document.querySelector('#app'),
);