CSS-only off-canvas menu, no javascript at all. Requires browser support for :checked pseudo-class and adjacent sibling selector. Fiddle at http://jsfiddle.net/glueckpress/3psut/.

css-only-off-canvas-menu.css

* {
    margin: 0;
    padding: 0;
}
html,
body,
#page {
    height: 100%;
}
p, ul {
    margin: 0 1em 1.5em;
}
ul {
    list-style: none;
    padding-left: 0;
}
h1 {
    background: #212121;
    color: #fff;
    font-family: sans-serif;
    font-size: 1em;
    font-weight: normal;
    line-height: 36px; /* measure against label */
    position: absolute;
    text-align: center;
    top: 0;
    width: 100%;
    z-index: 1;
}
#page {
    overflow: hidden;
    padding-top: 36px;
    position: relative;
}
label[for="menu-toggle"] {
    border-top: 12px double #fff;
    border-bottom: 4px solid #fff;
    cursor: pointer;
    height: 4px;
    left: 8px;
    overflow: hidden;
    text-indent: 100%;
    width: 36px;
    position: absolute;
    top: 8px;
    z-index: 1;
}
#menu-toggle { /* goes off the canvas */
    left: -100%;
    position: absolute;
    top: -100%;
    -webkit-appearance: none;
}
nav,
main {
    box-sizing: border-box;
    display: block;
    float: left;
    overflow: hidden;
    padding: 1em 0;
    position: relative;
    -webkit-transition: all .25s ease-in;
}
nav {
    background: #212121;
    clear: left;
    color: #fff;
    min-height: 100%;
    width: 30%;
}
nav a {
    color: #fff;
}
main {
    width: 70%;
}
/* 
 * For decent browsers only.
 * Delete to test fallback.
 */
#menu-toggle:not(:checked) ~ nav { 
    margin-left: -30%;
}
#menu-toggle:checked ~ nav {
    margin-left: 0;
}
#menu-toggle:not(:checked) ~ nav ~ main {
    width: 100%;
}
#menu-toggle:checked ~ nav ~ main {
    width: 70%;
}

css-only-off-canvas-menu.html

<div id="page">
    <h1>Site Title</h1>
    <label for="menu-toggle">Menu</label>
    <input id="menu-toggle" type="checkbox" name="menu-toggle">
    <nav>
        <ul>
            <li><a href="#">Menu item</a></li>
            <li><a href="#">Menu item</a></li>
            <li><a href="#">Menu item</a></li>
        </ul>
    </nav>
    <main>
        <p>Some fancy web site content goes here. But behold, press the three bars at the top and there shall be a menu!</p>
    </main>
</div>