checkPleaseArrowKeys.user.js

// ==UserScript==
// @name        Check, Please! Arrow Keys
// @namespace   https://gist.github.com/JulianWebb
// @version     0.2
// @author      Pongles
// @description Adds the ability to navigate the comic using just the left and right arrow keys
// @icon        https://checkpleasecomic.com/favicon.ico
// @match       https://checkpleasecomic.com/comic/*
// @grant       none
// ==/UserScript==

(function() {
  'use strict';
  
  console.log('Check, Please! Arrow Keys is working!')
  document.onkeydown = function(e) {
    switch (e.key) {
      case 'ArrowLeft':
        let prevInChapter = document.querySelector(".slick-prev");
        if (prevInChapter.className.includes("slick-disabled")) {
          let prevChapter = document.querySelector(".cc-prev");
          prevChapter.click()
        } else {
         prevInChapter.click() 
        }
        break;
      case 'ArrowRight': 
        let nextInChapter = document.querySelector(".slick-next");
        if (nextInChapter.className.includes("slick-disabled")) {
          let nextChapter = document.querySelector(".cc-next");
          nextChapter.click();
        } else {
          nextInChapter.click();   
        }
        break;
      default: return;
    }
  }
})();