example.css

html, body {
    padding: 0;
    margin: 0;
}


*, :after, :before {
  -webkit-box-sizing: border-box;
     -moz-box-sizing: border-box;
          box-sizing: border-box;
}

h1 {
    padding: 0 .5em;
}

.foo, .bar, .baz, .foobar {
    margin: 10px;
    padding: 10px;
    border-radius: 4px;
}

.foo {
    font-size: 11pt;
}
.bar {
    font-size: 13pt;
}
.baz {
    font-size: 16pt;
}

.parent-iframe {
    resize: both;
    margin: 3%;
    width: 94%;
    height: 200px;
    min-width: 100px;
    min-height: 100px;
    overflow: hidden;
    border: 1px solid #c1c1c1;
}

.parent-iframe > iframe {
    width: 100%;
    height: 100%;
    border: none;
}

button {
    background-color: #007bff;
    border-radius: 2px;
    border: 1px solid transparent;
    padding: 10px;
    margin: 2px;
    color: #fff;
    cursor: pointer;
}

textarea {
    width: 98%;
    height: 100px;
}

button:focus, button:active {
    box-shadow: 0 0 0 .2rem rgba(0, 123, 255, .5);
    background-color: #0069d9;
    border-color: #001123;
}

.design-mode {
    border: 1px solid #0c0c0c;
    background-color: #fff;
    width: 480px;
    height: 320px;
    resize: both;
}

.paste-here {
    margin: 1em;
    border-radius: .2em;
    position: relative;
    border: thin solid #c1c1c1;
    background: rgb(245,239,241);
    background: linear-gradient(0deg, rgba(245,239,241,.5) 0%, rgba(214,209,209,.5) 100%);
    box-shadow: 0 0 5px rgba(0,0,0,.1) inset;
}

.paste-here:after {
    background: rgba(0,0,0,.5);
    position: absolute;
    bottom: 10px;
    right: 10px;
    color: #fff;
    border-radius: 3px;
    padding: 5px 10px;
    content: "paste here";
    text-transform: uppercase;
}

.paste-here > div {
    border-radius: inherit;
    border: none;
    outline: none;
    overflow: auto;
    padding: 1em;
    width: 100%;
    height: 320px;
    box-shadow: 0 0 .08em .5em rgba(0,0,255,0);
    transition: all .2s ease-out;
}

.paste-here > div:focus {
    box-shadow: 0 0 .08em .4em rgba(0,0,255,.2);
}

code.source {
    margin-top: 1em;
    background: #161a42;
    color: #fff;
    padding: .8em 1.5em;
    border-radius: .2em;
    display: block;
}