codeacademy.com enhancements. Currently changes the text opacity of unfinished tasks and also adds additional keyboard shortcuts (Alt-LeftArrow and Alt-RightArrow) for Back and Next.

codeacademy.com_enhanced.user.js

// ==UserScript==
// @name codeacademy.com enhancements
// @namespace Leonard Högström
// @match https://www.codecademy.com/courses/*
// @grant none
// ==/UserScript==

// Change the text opacity of unchecked tasks for better readability
// so you can do all of them at once instead of having to run the code with every completed task.
[].filter.call(document.querySelectorAll('.fcn-checkpoint--disabled .fcn-checkpoint__body'),item => item)
  .forEach(item => item.style.opacity = 1);

// Add keyboard shortcuts to the 'Back' and 'Next' buttons for easier use with non-english keyboards.
// Alt-LeftArrow and Alt-RightArrow
window.addEventListener("keydown", function(e) {
  if (e.ctrlKey) {
    console.log(e.altKey)
    if (e.which === 39) {
      console.log('next')
      var _ = document.querySelector('#discovery-next')
      _.disabled || _.click();      
    }
    if (e.which === 37) {
      console.log('back')
      var _ = document.querySelector('.fcn-nav-wrapper > div > button')
      _.innerText === 'Back' && _.click();
    }
  }
}, false);