Tampermonkey script - Adds keyboard shortcuts for easily navigating the Homestuck webcomic. May work for other MSPA comics.

homestuck-keyboard.user.js

// ==UserScript==
// @name         Homestuck Keyboard Advancer
// @namespace    http://matt.scharley.me/
// @version      1.13
// @updateURL    https://gist.githubusercontent.com/mscharley/2d73c81d1d70c504d58d/raw/homestuck-keyboard.user.js
// @downloadURL  https://gist.githubusercontent.com/mscharley/2d73c81d1d70c504d58d/raw/homestuck-keyboard.user.js
// @description  Adds keyboard shortcuts for easily navigating the Homestuck webcomic. May work for other MSPA comics.
// @author       Matthew Scharley
// @match        *://www.homestuck.com/*
// @grant        none
// @run-at       document-end
// ==/UserScript==

// Click the raw button above to install this script.

(function() {
    "use strict";
    var backLink = null,
        nextLink = null;

    var comicAreas = document.querySelectorAll(
        // Regular coloring
        'td[bgcolor="c6c6c6"],td[bgcolor="#c6c6c6"],' +
        // Doc Scratch coloring (scratch.php)
        'td[bgcolor="0E4603"],td[bgcolor="#0E4603"],' +
        // Trickster coloring (trickster.php)
        'td[bgcolor="FF73FD"],td[bgcolor="#FF73FD"],' +
        // Act 6 Act 6 coloring (ACT6ACT6.php)
        'td[bgcolor="073C00"],td[bgcolor="#073C00"],' +
        // New website design
        'div.row'
    );

    var links = nodeListQuerySelectorAll(comicAreas, 'a[href^="?s="],a[href^="/story/"]');

    for (var i in links) {
        if (!links[i].href) {
            continue;
        }

        var parent = links[i].parentNode;
        if (parent.nodeName == 'FONT' && parent.size == '5') {
            nextLink = links[i];
        } else if (parent.parentNode.classList.contains("o_story-nav")) {
            nextLink = links[i];
        } else if (links[i].innerHTML == 'Go Back') {
            backLink = links[i];
        }
    }
    if (nextLink == null) {
        console.log("hka: unable to find a next link.");
    } else { console.log("hka: next link", nextLink); }
    if (backLink == null) {
        console.log("hka: unable to find a back link.");
    } else { console.log("hka: back link", backLink); }

    document.addEventListener('keyup', function(event) {
        var flash_elements = nodeListQuerySelectorAll(comicAreas, '[type="application/x-shockwave-flash"], canvas');
        var has_flash = flash_elements.length;
        if (has_flash) {
            console.log("hka: not adding listeners to this page because it looks like an [S]");
            return;
        }

        switch(event.code) {
            case "ArrowRight":
                event.preventDefault();

                console.log("hka: trying to click", nextLink);
                if (nextLink) {
                    fireEvent(nextLink, 'click');
                }

                break;
            case "ArrowLeft":
                event.preventDefault();

                console.log("hka: trying to click", backLink);
                if (backLink) {
                    fireEvent(backLink, 'click');
                }

                break;
            case "KeyP":
                // P
                event.preventDefault();

                var spoilers = document.querySelectorAll('.o_chat-log-btn');
                for (var i in spoilers) {
                    fireEvent(spoilers[i], 'click');
                }

                break;
        }
    });

    /**
     * @param {NodeList} list A nodelist or array of Nodes to search
     * @param {String} selector A selector to apply to all elements
     */
    function nodeListQuerySelectorAll(list, selector) {
        var mappings = [];

        for (var i = 0; i < list.length; i++) {
            var sublisting = list[i].querySelectorAll(selector);

            for (var j = 0; j < sublisting.length; j++) {
                mappings.push(sublisting[j]);
            }
        }

        return mappings;
    }

    /**
     * Fire an event handler to the specified node. Event handlers can detect that the event was fired programatically
     * by testing for a 'synthetic=true' property on the event object.
     *
     * @param {HTMLNode} node The node to fire the event handler on.
     * @param {String} eventName The name of the event without the "on" (e.g., "focus")
     *
     * @see http://stackoverflow.com/a/2381862/15537
     */
    function fireEvent(node, eventName) {
        // Make sure we use the ownerDocument from the provided node to avoid cross-window problems
        var doc;
        if (node.ownerDocument) {
            doc = node.ownerDocument;
        } else if (node.nodeType == 9){
            // the node may be the document itself, nodeType 9 = DOCUMENT_NODE
            doc = node;
        } else {
            throw new Error("Invalid node passed to fireEvent: " + node.id);
        }

         if (node.dispatchEvent) {
            // Gecko-style approach (now the standard) takes more work
            var eventClass = "";

            // Different events have different event classes.
            // If this switch statement can't map an eventName to an eventClass,
            // the event firing is going to fail.
            switch (eventName) {
                case "click": // Dispatching of 'click' appears to not work correctly in Safari. Use 'mousedown' or 'mouseup' instead.
                case "mousedown":
                case "mouseup":
                    eventClass = "MouseEvents";
                    break;

                case "focus":
                case "change":
                case "blur":
                case "select":
                    eventClass = "HTMLEvents";
                    break;

                default:
                    throw "fireEvent: Couldn't find an event class for event '" + eventName + "'.";
            }
            var event = doc.createEvent(eventClass);

            var bubbles = eventName == "change" ? false : true;
            event.initEvent(eventName, bubbles, true); // All events created as bubbling and cancelable.

            event.synthetic = true; // allow detection of synthetic events
            // The second parameter says go ahead with the default action
            node.dispatchEvent(event, true);
        } else  if (node.fireEvent) {
            // IE-old school style
            var event = doc.createEventObject();
            event.synthetic = true; // allow detection of synthetic events
            node.fireEvent("on" + eventName, event);
        }
    }
})();