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KAnki is designed for jailbroken Kindle devices with very limited browser capabilities. Understanding these constraints is essential for customizing the app or creating compatible decks.

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Kindle browser constraints

Kindle devices run an older WebKit-based browser with severely limited features compared to modern browsers.

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JavaScript version

KAnki must use ES5 JavaScript only. No modern JavaScript features are supported.

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Not available

  • Arrow functions: () => {}
  • Template literals: `Hello ${name}`
  • Destructuring: const {x, y} = obj
  • Spread operator: ...array
  • let and const declarations
  • Classes: class MyClass {}
  • Promises and async/await
  • Array methods like .find(), .includes(), .from()
  • Map, Set, Symbol, WeakMap

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Must use instead

  • var for all variable declarations
  • function keyword for functions
  • String concatenation: 'Hello ' + name
  • Array.prototype.indexOf() instead of .includes()
  • Callback-based patterns instead of promises
  • XMLHttpRequest instead of fetch API

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ES5 patterns in KAnki source

Here are examples from the actual codebase: 
// Variable declarations - only 'var' works
var currentCardIndex = 0;
var correctAnswers = 0;
var deck = null;

// Function declarations - no arrow functions
function getDueCards() {
  var now = new Date().getTime();
  var dueCards = [];
  
  for (var i = 0; i < deck.cards.length; i++) {
    var card = deck.cards[i];
    if (card.nextReview <= now) {
      dueCards.push(card);
    }
  }
  
  return dueCards;
}

// Closures for event handlers
function createLevelChangeHandler(level) {
  return function() {
    changeLevel(level);
  };
}

// Array iteration - no forEach or map
for (var i = 0; i < deck.cards.length; i++) {
  deck.cards[i].difficulty = 0;
}

// Filtering arrays manually
incorrectCardsQueue = incorrectCardsQueue.filter(function(card) {
  return card !== null;
});

// Timeouts use function() syntax
setTimeout(function() {
  fontLoaded = true;
  displayCurrentCard(false);
}, 1000);

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Polyfills included

KAnki includes polyfill.min.js to provide some modern JavaScript features as ES5-compatible implementations. However, the core codebase avoids relying on these when possible.

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CSS limitations

The Kindle browser supports only basic CSS2 with minimal CSS3 features.

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Not available

Modern layout systems don’t work on Kindle browsers.
  • No Flexbox: display: flex is not supported
  • No CSS Grid: display: grid is not supported
  • No CSS variables: var(--color-primary) doesn’t work
  • Limited transforms: Most transform properties fail
  • No animations: @keyframes and animation are unreliable
  • No media queries: @media has very limited support
  • No pseudo-elements: :before and :after work inconsistently

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What works

/* Basic box model */
body {
  margin: 0;
  padding: 0;
  background-color: #f5f5f5;
}

/* Absolute and relative positioning */
.popup {
  position: absolute;
  top: 50%;
  left: 50%;
  margin-top: -100px;
  margin-left: -140px;
}

/* Table-based layouts (most reliable) */
.menuBar {
  width: 98%;
  display: table;
  table-layout: fixed;
  border-spacing: 0;
}

.menuSection {
  display: table-cell;
  vertical-align: middle;
  text-align: center;
}

/* Basic typography */
.cardFront {
  font-size: 2em;
  font-weight: bold;
  font-family: 'LanguageFont', sans-serif;
}

/* Fixed dimensions */
.card {
  height: 300px;
  overflow-y: auto;
  overflow-x: hidden;
}

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Layout strategy

From the KAnki source, the app uses:
  1. Table-based layouts for complex arrangements:
.topBar {
  width: 100%;
  display: table;
  table-layout: fixed;
}

.logoArea {
  display: table-cell;
  width: 25%;
}

.infoArea {
  display: table-cell;
  width: 75%;
}
  1. Fixed heights to prevent layout shifts on e-ink displays:
.card {
  height: 300px; /* Fixed height instead of min-height */
}

#controlButtons {
  height: 100px;
  position: relative;
}
  1. Absolute positioning for overlays:
#intervalButtons {
  position: absolute;
  top: 0;
  left: 0;
  right: 0;
}

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Font loading

Custom fonts must use @font-face with TTF format: 
@font-face {
  font-family: 'LanguageFont';
  src: url('assets/fonts/language.ttf') format('truetype');
  font-weight: normal;
  font-style: normal;
  font-display: block;
}

body {
  font-family: 'LanguageFont', sans-serif;
}
KAnki waits 1000ms after page load to ensure fonts are loaded before displaying cards. This prevents rendering issues with non-Latin scripts.

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E-ink display optimization

E-ink screens refresh slowly and can show ghosting. KAnki implements several techniques to minimize visual issues:

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Fixed element heights

function initializeFixedHeights() {
  var cardContainer = document.getElementById("cardContainer");
  var controlButtons = document.getElementById("controlButtons");
  
  cardContainer.style.height = "300px";
  controlButtons.style.height = "100px";
  
  // Prevents layout shifts during content changes
}

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Visibility instead of display

// Hide without changing layout
intervalButtons.style.visibility = "hidden";

// Show when needed
intervalButtons.style.visibility = "visible";
This prevents the screen from reflowing when elements appear/disappear.

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Grayscale enforcement

body {
  filter: grayscale(1);
}
Forces grayscale rendering since Kindle e-ink displays don’t show color.

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Screen size variations

Different Kindle models have vastly different resolutions:
  • Basic Kindle: 600×800 pixels
  • Paperwhite: 1072×1448 pixels (higher DPI)
  • Oasis: 1264×1680 pixels
  • Scribe: 1860×2480 pixels

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Responsive scaling

KAnki detects screen size and applies device-specific scaling: 
function detectDeviceAndSetScaling() {
  var width = window.innerWidth;
  var height = window.innerHeight;
  var deviceScaleFactor = 1.0;
  
  // Paperwhite detection
  if ((width >= 1070 && width <= 1080) && 
      (height >= 1440 && height <= 1460)) {
    deviceScaleFactor = 0.6;
  }
  // High DPI devices
  else if (width >= 1000 && height >= 1400) {
    deviceScaleFactor = 0.65;
  }
  // Mid-size screens
  else if (width >= 750 && width < 1000) {
    deviceScaleFactor = 0.8;
  }
  
  // Apply scaling
  document.documentElement.style.fontSize = 
    (deviceScaleFactor * 100) + "%";
}

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Size-specific CSS classes

The app adds classes to enable device-specific styling: 
if (width <= 600) {
  body.classList.add('kindle-small');
} else if (width <= 850) {
  body.classList.add('kindle-medium');
} else if (width <= 1300) {
  body.classList.add('kindle-large');
} else {
  body.classList.add('kindle-xlarge');
}

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Storage limitations

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localStorage only

Kindle browsers don’t support IndexedDB, Web SQL, or other modern storage APIs.
KAnki uses localStorage exclusively: 
// Saving data
function saveDeck() {
  if (deck) {
    try {
      localStorage.setItem('kanki_deck', JSON.stringify(deck));
    } catch (e) {
      log("Error saving deck: " + e.message);
    }
  }
}

// Loading data
function loadDeck() {
  try {
    var savedDeck = localStorage.getItem('kanki_deck');
    if (savedDeck) {
      deck = JSON.parse(savedDeck);
      return true;
    }
  } catch (e) {
    log("Error loading deck: " + e.message);
  }
  return false;
}

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Storage location

Data is stored at: 
/Kindle/.active_content_sandbox/kanki/resource/LocalStorage/file__0.localstorage
localStorage has a size limit of approximately 5-10MB on Kindle devices. For large decks (1000+ cards with extensive history), this can become a constraint.

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Performance considerations

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Debouncing viewport changes

function handleViewportChange() {
  if (window.resizeTimer) {
    clearTimeout(window.resizeTimer);
  }
  
  window.resizeTimer = setTimeout(function() {
    initializeFixedHeights();
    displayCurrentCard(false);
  }, 250);
}

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Preventing accidental inputs

function handleAnswerWithInterval(difficulty) {
  // Prevent button presses within 500ms of showing answer
  if (Date.now() - lastShowAnswerTime < 500) {
    return;
  }
  // ... process answer
}
E-ink displays refresh slowly, so rapid interactions can cause problems.

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DOM updates

Reuse existing elements instead of recreating: 
function displayCurrentCard(showAnswer) {
  // Get existing elements
  var frontElement = document.getElementById("cardFront");
  var backElement = document.getElementById("cardBack");
  
  // Update content
  frontElement.innerHTML = card.front;
  backElement.textContent = card.back;
  
  // Toggle visibility
  backElement.style.display = showAnswer ? "block" : "none";
}

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Development workflow

When customizing KAnki:
  1. Test on actual hardware: Kindle’s browser behavior differs significantly from desktop browsers
  2. Use ES5 syntax exclusively: Run code through an ES5 transpiler if needed
  3. Avoid modern CSS: Stick to table layouts and absolute positioning
  4. Minimize DOM manipulation: Batch updates and reuse elements
  5. Test all Kindle models: Screen sizes vary dramatically
While you could use Babel to transpile modern JavaScript to ES5, the KAnki source code is deliberately written in ES5 to avoid build steps and keep development simple. If you add transpilation, test thoroughly on actual Kindle hardware as some polyfills may not work correctly.
Progressive enhancement would add complexity without benefit. Since the target platform (Kindle) is well-defined and unchanging, optimizing specifically for Kindle’s capabilities produces the most reliable results.

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