Edit in JSFiddle

Result
JavaScript
CSS
HTML

/*!
 * avgrund 0.1
 * http://lab.hakim.se/avgrund
 * MIT licensed
 *
 * Copyright (C) 2012 Hakim El Hattab, http://hakim.se
 */
(function(){

    var container = document.documentElement,
        popup = document.querySelector( '.avgrund-popup' ),
        cover = document.querySelector( '.avgrund-cover' ),
        currentState = null;

    container.className = container.className.replace( /\s+$/gi, '' ) + ' avgrund-ready';

    // Deactivate on ESC
    function onDocumentKeyUp( event ) {
        if( event.keyCode === 27 ) {
            deactivate();
        }
    }

    // Deactivate on click outside
    function onDocumentClick( event ) {
        if( event.target === cover ) {
            deactivate();
        }
    }

    function activate( state ) {
        document.addEventListener( 'keyup', onDocumentKeyUp, false );
        document.addEventListener( 'click', onDocumentClick, false );

        removeClass( popup, currentState );
        addClass( popup, 'no-transition' );
        addClass( popup, state );

        setTimeout( function() {
            removeClass( popup, 'no-transition' );
            addClass( container, 'avgrund-active' );
        }, 0 );

        currentState = state;
    }

    function deactivate() {
        document.removeEventListener( 'keyup', onDocumentKeyUp, false );
        document.removeEventListener( 'click', onDocumentClick, false );

        removeClass( container, 'avgrund-active' );
    }

    function disableBlur() {
        addClass( document.documentElement, 'no-blur' );
    }

    function addClass( element, name ) {
        element.className = element.className.replace( /\s+$/gi, '' ) + ' ' + name;
    }

    function removeClass( element, name ) {
        element.className = element.className.replace( name, '' );
    }

    window.avgrund = {
        activate: activate,
        deactivate: deactivate,
        disableBlur: disableBlur
    }

})();

/*!
 * avgrund 0.1
 * http://lab.hakim.se/avgrund
 * MIT licensed
 *
 * Created by Hakim El Hattab, http://hakim.se
 */

* {
    margin: 0;
    padding: 0;
}

html, 
body {
    height: 100%;
    overflow: hidden;
}

html {
    background-color: #222;
    background-image: url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAQAAAAECAYAAACp8Z5+AAAAGklEQVQIW2NkYGD4D8SMQAwGcAY2AbBKDBUAVuYCBQPd34sAAAAASUVORK5CYII=);
    background-repeat: repeat;
}

h1, 
h2 {
    font-size: 24px;
}

p {
    margin: 10px 0 10px 0;
    font-size: 16px;
    line-height: 1.32;
}

a {
    color: #7aa76d;
    text-decoration: none;

    -webkit-transition: 0.15s color ease;
       -moz-transition: 0.15s color ease;
        -ms-transition: 0.15s color ease;
         -o-transition: 0.15s color ease;
            transition: 0.15s color ease;
}
    a:hover {
        color: #91cd85;
    }

small {
    display: block;
    margin-top: 15px;
    padding-top: 15px;
    color: #333;
    font-size: 0.85em;
    border-top: 1px dashed #ccc;

    -webkit-text-size-adjust: none;
}

button {
    border: 0px;
    padding: 8px 10px;
    margin: 5px 0px;
    border-radius: 1px;

    cursor: pointer;
    color: #fff;
    background: #7aa76d;
    font-size: 15px;

    -webkit-transition: 0.15s background ease;
       -moz-transition: 0.15s background ease;
        -ms-transition: 0.15s background ease;
         -o-transition: 0.15s background ease;
            transition: 0.15s background ease;
}
    button:hover {
        background: #91cd85;
    }
    button:active {
        background: #60895a;
    }
    button+button {
        margin-left: 5px;
    }

.sharing {
    margin-top: 50px;
}

body {
    background: #fff;

    font-family: 'Lato', Helvetica, sans-serif;
    font-size: 16px;
    color: #222;
}
    .avgrund-active body {
        -webkit-transform: scale( 0.9 );
           -moz-transform: scale( 0.9 );
            -ms-transform: scale( 0.9 );
             -o-transform: scale( 0.9 );
                transform: scale( 0.9 );
    }

.avgrund-cover {
    position: absolute;
    width: 100%;
    height: 100%;
    top: 0;
    left: 0;
    z-index: 1;
    visibility: hidden;
    opacity: 0;
    background: rgba( 0, 0, 0, 0.5 );
}
    .avgrund-active .avgrund-cover {
        visibility: visible;
        opacity: 1;
    }

.avgrund-contents {
    position: relative;
    padding: 20px;
    max-width: 400px;
    height: 100%;
    margin: auto;
}
    .avgrund-active .avgrund-contents {
        -webkit-filter: blur(2px);
           -moz-filter: blur(2px);
            -ms-filter: blur(2px);
             -o-filter: blur(2px);
                filter: blur(2px);
    }
    .no-blur.avgrund-active .avgrund-contents {
        -webkit-filter: none;
           -moz-filter: none;
            -ms-filter: none;
             -o-filter: none;
                filter: none;
    }

.avgrund-popup {
    position: absolute;
    width: 340px;
    height: 180px;
    left: 50%;
    top: 50%;
    margin: -130px 0 0 -190px;
    visibility: hidden;
    opacity: 0;
    z-index: 2;
    padding: 20px;

    background: white;
    box-shadow: 0px 0px 20px rgba( 0, 0, 0, 0.6 );
    border-radius: 3px;

    -webkit-transform: scale( 0.8 );
       -moz-transform: scale( 0.8 );
        -ms-transform: scale( 0.8 );
         -o-transform: scale( 0.8 );
            transform: scale( 0.8 );
}
    .avgrund-active .avgrund-popup {
        visibility: visible;
        opacity: 1;

        -webkit-transform: scale( 1.1 );
           -moz-transform: scale( 1.1 );
            -ms-transform: scale( 1.1 );
             -o-transform: scale( 1.1 );
                transform: scale( 1.1 );
    }
    .avgrund-popup.stack {
        -webkit-transform: scale( 1.5 );
           -moz-transform: scale( 1.5 );
            -ms-transform: scale( 1.5 );
             -o-transform: scale( 1.5 );
                transform: scale( 1.5 );
    }
    .avgrund-active .avgrund-popup.stack {
        -webkit-transform: scale( 1.1 );
           -moz-transform: scale( 1.1 );
            -ms-transform: scale( 1.1 );
             -o-transform: scale( 1.1 );
                transform: scale( 1.1 );
    }


.avgrund-ready body,
.avgrund-ready .avgrund-contents,
.avgrund-ready .avgrund-popup,
.avgrund-ready .avgrund-cover {
    -webkit-transform-origin: 50% 50%;
       -moz-transform-origin: 50% 50%;
        -ms-transform-origin: 50% 50%;
         -o-transform-origin: 50% 50%;
            transform-origin: 50% 50%;

    -webkit-transition: 0.3s all cubic-bezier(0.250, 0.460, 0.450, 0.940);
       -moz-transition: 0.3s all cubic-bezier(0.250, 0.460, 0.450, 0.940);
        -ms-transition: 0.3s all cubic-bezier(0.250, 0.460, 0.450, 0.940);
         -o-transition: 0.3s all cubic-bezier(0.250, 0.460, 0.450, 0.940);
            transition: 0.3s all cubic-bezier(0.250, 0.460, 0.450, 0.940);
}
.avgrund-ready .avgrund-popup.no-transition {
    -webkit-transition: none;
       -moz-transition: none;
        -ms-transition: none;
         -o-transition: none;
            transition: none;
}

    <article class="avgrund-contents">

            <button onclick="avgrund.activate( 'stack' );">foo</button>
            <button onclick="avgrund.activate();">bar</button>


        </article>

        <div class="avgrund-cover"></div>

        <aside class="avgrund-popup">
            <h2>Foo!Bar!</h2>
            <p>
               hogehoge
            </p>

            <button onclick="avgrund.deactivate();">X</button>
        </aside>