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 (Alt-K) 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");

        InsertKbdTag (targArea[0]);
        targArea.focus ();
        try {
            //--- This is a utility function that SE currently provides on its pages.
            StackExchange.MarkdownEditor.refreshAllPreviews ();
        }
        catch (e) {
            console.warn ("***Userscript error: refreshAllPreviews() is no longer defined!");
        }
    }

    function InsertKbdTag (node) {
        //--- Wrap selected text or insert at curser.
        var tagLength       = 5;    // Tag is: "<kbd>"
        var oldText         = node.value || node.textContent;
        var newText;
        var iTargetStart    = node.selectionStart;
        var iTargetEnd      = node.selectionEnd;
        var selectedText    = oldText.slice (iTargetStart, iTargetEnd);
        var possWrappedTxt;
        try {
            //--- Lazyman's overrun checking...
            possWrappedTxt  = oldText.slice (
                                iTargetStart - tagLength,
                                iTargetEnd   + tagLength + 1
                            );
        }
        catch (e) {
            possWrappedTxt  = "Text can't be wrapped, cause we overran the string.";
        }

        /*--- Is the current selection wrapped?  If so, just unwrap it.
            This works the same way as SE's bold, italic, code, etc...
            "]text["                --> "<kbd>]text[</kbd>"
            "<kbd>]text[</kbd>"     --> "]text["
            "]<kbd>text</kbd>["     --> "<kbd>]<kbd>text</kbd>[</kbd>"

            Except that:
            "]["                    --> "<kbd>][</kbd>"
            "<kbd>][</kbd>"         --> "]["
            with no placeholder text.
            Note that `]` and `[` denote the selected text here.
        */
        if (possWrappedTxt  &&
            selectedText    == possWrappedTxt.replace (/^<kbd>((?:.|\n|\r)*)<\/kbd>$/, "$1")
        ) {
            iTargetStart   -= tagLength;
            iTargetEnd     += tagLength + 1;
            newText         = oldText.slice (0, iTargetStart)
                            + selectedText + oldText.slice (iTargetEnd)
                            ;
            iTargetEnd      = iTargetStart + selectedText.length;
        }
        else {
            /*--- Here we will wrap the selection in our tags, but there is one extra
                condition.  We don't want to wrap leading or trailing whitespace.
            */
            var trimSelctd  = selectedText.match (/^(\s*)(\S?(?:.|\n|\r)*\S)(\s*)$/)
                            || ["", "", "", ""]
                            ;
            if (trimSelctd.length != 4) {
                console.warn ("***Userscript error: unexpected failure of whitespace RE.");
            }
            else {
                newText         = trimSelctd[1]     //-- Leading whitespace, if any.
                                + '<kbd>' + trimSelctd[2] + '</kbd>'
                                + trimSelctd[3]     //-- Trailing whitespace, if any.
                                ;
                newText         = oldText.slice (0, iTargetStart)
                                + newText + oldText.slice (iTargetEnd)
                                ;
                iTargetStart   += tagLength + trimSelctd[1].length;
                iTargetEnd     += tagLength - trimSelctd[3].length;
            }
        }

        //console.log (newText);
        node.value          = newText;
        //--- After using spelling corrector, this gets buggered, hence the multiple sets.
        node.textContent    = newText;

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


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);
};