実装：
Hキーを押すとポップアップを表示。もう一度押すと非表示。

HTML

<div class="hover_bkgr_fricc">
        <span class="helper"></span>
        <div>
            // HTML here
        </div>
</div>

CSS

/* # ヘルプポップアップ */
/* Popup box BEGIN */
.hover_bkgr_fricc{
    background:rgba(0,0,0,.4);
    cursor:pointer;
    display:none;
    height:100%;
    position:fixed;
    text-align:center;
    top:0;
    width:100%;
    z-index:10000;
}
.hover_bkgr_fricc .helper{
    display:inline-block;
    height:100%;
    vertical-align:middle;
}
.hover_bkgr_fricc > div {
    background-color: #fff;
    box-shadow: 10px 10px 60px #555;
    display: inline-block;
    height: auto;
    max-width: 551px;
    min-height: 100px;
    vertical-align: middle;
    width: 60%;
    position: relative;
    border-radius: 8px;
    padding: 15px 5%;
}
.popupCloseButton {
    background-color: #fff;
    border: 3px solid #999;
    border-radius: 50px;
    cursor: pointer;
    display: inline-block;
    font-family: arial;
    font-weight: bold;
    position: absolute;
    top: -20px;
    right: -20px;
    font-size: 25px;
    line-height: 30px;
    width: 30px;
    height: 30px;
    text-align: center;
}
.popupCloseButton:hover {
    background-color: #ccc;
}
.trigger_popup_fricc {
    cursor: pointer;
    font-size: 20px;
    margin: 20px;
    display: inline-block;
    font-weight: bold;
}
/* Popup box BEGIN */

JavaScript

//# ヘルプポップアップ


var showPopupToggle = "no"

// H to popup help window
function showPopup(e) {
    if (e.keyCode == 72) {
      if (showPopupToggle == "no"){
        $('.hover_bkgr_fricc').show();
        showPopupToggle = "yes"
      } else {
        $('.hover_bkgr_fricc').hide();
        showPopupToggle = "no"
      }
    }
}
document.addEventListener('keyup', showPopup, false);