Edit in JSFiddle

<div class="editor fix-height">
    <header class="informations fix-width">
        <input id="subject" class="fix-width" type="text" value="" placeholder="Subject" />
    </header>
    <div class="main fix-height">
        <textarea id="body" class="fix-height fix-width" placeholder="Body"></textarea>
    </div>
    <footer class="status fix-width">
        <div id="status-message">Ready.</div>
    </footer>
</div>

/* Base */
html,body { height:100%; margin:0; }

/* Common */
.fix-height {
    box-sizing: border-box;
    height: 100%;
}
.fix-width {
    box-sizing: border-box;
    width: 100%;
}

/* General */
.editor {
    margin: 0;
    padding: 30px 0 20px;
}

/* Informations */
.informations {
    height: 20px;
    left: 0;
    position: absolute;
    top: 0;
}
.informations input {
    height: 30px;
}

/* Body */
.main {
    height: 100%;
}
#body {
    resize: none;
}

/* Status */
.status {
    background-color: #eee;
    bottom: 0;
    color: #666;
    font-size: 12px;
    height: 20px;
    left: 0;
    line-height: 16px;
    padding: 2px 4px;
    position: absolute;
    text-shadow: 1px 1px #fff;
    width: 100%;
}