This is a userscript that adds shortcuts for adding <kbd> tags to posts. Designed for Stack Exchange sites.

Add_kbd_shortcut.user.js

// ==UserScript==
// @name            _Add kbd shortcut
// @namespace       StackExchange
// @description     Adds a button and a keyboard shortcut to add <kbd> tags.
// @match           http://*.askubuntu.com/*
// @match           http://*.onstartups.com/*
// @match           http://*.serverfault.com/*
// @match           http://*.stackapps.com/*
// @match           http://*.stackexchange.com/*
// @match           http://*.stackoverflow.com/*
// @match           http://*.superuser.com/*
// ==/UserScript==

function AddKbdShortcuts ($) {
    $("textarea.wmd-input").each (AddKbdButtonAsNeeded);

    //.on() not working right!!
    $("textarea.wmd-input").live ("focus",   AddKbdButtonAsNeeded);
    $("textarea.wmd-input").live ("keydown", InsertKbdTagByKeypress);
    $("li.wmd-kbd-button") .live ("click",   InsertKbdTagByClick);

    /*------------*/
    function AddKbdButtonAsNeeded () {
        var jThis   = $(this);
        if ( ! jThis.data ("hasKbdBttn") ) {
            //--- Find the button bar and add our button.
            var btnBar  = jThis.prevAll ("div.wmd-button-bar");
            if (btnBar.length) {
                //--- The button bar takes a while to AJAX-in.
                var bbListTimer = setInterval ( function() {
                        var bbList  = btnBar.find ("ul.wmd-button-row");
                        if (bbList.length) {
                            clearInterval (bbListTimer);
                            bbList.append (
                                  '<li class="wmd-button wmd-kbd-button" '
                                + 'title="Keyboard tag &lt;kbd&gt; Alt+K" '
                                + 'style="left: 380px;">'
                                + '<span style="background: white;">[kbd]</span></li>'
                            );
                            jThis.data ("hasKbdBttn", true);
                        }
                    },
                    100
                );
            }
        }
    }

    function InsertKbdTagByKeypress (zEvent) {
        //--- On Alt-K, insert the <kbd> set. Ignore all other keys.
        if (zEvent.altKey  &&  zEvent.which == 75) {
            InsertKbdTag (this);
            return false;
        }
        return true;
    }

    function InsertKbdTagByClick (zEvent) {
        //--- From the clicked button, find the matching textarea.
        var targArea    = $(this).parents ("div.wmd-button-bar")
                        .nextAll ("textarea.wmd-input");
        /*--- Due to SE's js, it's better to send a keypress event, instead of
            calling InsertKbdTag directly.
            //InsertKbdTag (targArea[0]);
        */
        targArea.trigger ( {
            type:   "keydown",
            which:  75,
            altKey: true
        } );
        targArea.focus ();
    }

    function InsertKbdTag (node) {
        //--- Wrap selected text or insert at curser.
        var oldText         = node.value || node.textContent;
        var newText;
        var iTargetStart    = node.selectionStart;
        var iTargetEnd      = node.selectionEnd;

        if (iTargetStart == iTargetEnd){
            newText         = '<kbd></kbd>';
        }else{

            newText         = '<kbd>' + oldText.slice (iTargetStart, iTargetEnd) + '</kbd>';
        }
        //console.log (newText);
        if(oldText.slice (iTargetStart, iTargetEnd).match(/\<kbd\>/)&&oldText.slice (iTargetStart, iTargetEnd).match(/\<\/kbd\>/)){
             //--In case of "<kbd>text</kbd>" being selected
             newText=oldText.slice (0, iTargetStart)+oldText.slice (iTargetStart+5, iTargetEnd-6)+ oldText.slice (iTargetEnd)
             node.value          = newText;
             //-- After using spelling corrector, this gets buggered, hence the multiple sets.
             node.textContent    = newText;
 
             //-- Have to reset selection, since we repasted the text.
             node.selectionStart = iTargetStart;
             node.selectionEnd   = iTargetEnd-11;
        }else if(oldText.slice (iTargetStart-5, iTargetStart).match(/\<kbd\>/)&&oldText.slice (iTargetEnd, iTargetEnd+6).match(/\<\/kbd\>/)){
            //-- In case of "<kbd>[selected text]</kbd>"
            newText=oldText.slice (0, iTargetStart-5)+oldText.slice (iTargetStart, iTargetEnd)+ oldText.slice (iTargetEnd+6)
           node.value          = newText;
           //-- After using spelling corrector, this gets buggered, hence the multiple sets.
           node.textContent    = newText;

           //-- Have to reset selection, since we repasted the text.
           node.selectionStart = iTargetStart-5;
           node.selectionEnd   = iTargetEnd-5;
        }else{
           //No kbd's nearby
           newText             = oldText.slice (0, iTargetStart) + newText + oldText.slice (iTargetEnd);
           node.value          = newText;
           //-- After using spelling corrector, this gets buggered, hence the multiple sets.
           node.textContent    = newText;

           //-- Have to reset selection, since we repasted the text.
           node.selectionStart = iTargetStart + 5;
           node.selectionEnd   = iTargetEnd   + 5;
        }
   
    }
}


withPages_jQuery (AddKbdShortcuts);


function withPages_jQuery (NAMED_FunctionToRun) {
    //--- Use named functions for clarity and debugging...
    var funcText        = NAMED_FunctionToRun.toString ();
    var funcName        = funcText.replace (/^function\s+(\w+)\s*\((.|\n|\r)+$/, "$1");
    var script          = document.createElement ("script");
    script.textContent  = funcText + "\n\n";
    script.textContent += 'jQuery(document).ready( function () {' + funcName + '(jQuery);} );';
    document.body.appendChild (script);
};