diff --git a/ext/js/dom/document-focus-controller.js b/ext/js/dom/document-focus-controller.js
index bc6c61b9..d6c24646 100644
--- a/ext/js/dom/document-focus-controller.js
+++ b/ext/js/dom/document-focus-controller.js
@@ -22,11 +22,19 @@
  * focus a dummy element inside the main content, which gives keyboard scroll focus to that element.
  */
 class DocumentFocusController {
+    /**
+     * Creates a new instance of the class.
+     * @param {?string} autofocusElementSelector A selector string which can be used to specify an element which
+     *   should be automatically focused on prepare.
+     */
     constructor(autofocusElementSelector=null) {
         this._autofocusElement = (autofocusElementSelector !== null ? document.querySelector(autofocusElementSelector) : null);
         this._contentScrollFocusElement = document.querySelector('#content-scroll-focus');
     }
 
+    /**
+     * Initializes the instance.
+     */
     prepare() {
         window.addEventListener('focus', this._onWindowFocus.bind(this), false);
         this._updateFocusedElement(false);
@@ -35,6 +43,10 @@ class DocumentFocusController {
         }
     }
 
+    /**
+     * Removes focus from a given element.
+     * @param {Element} element The element to remove focus from.
+     */
     blurElement(element) {
         if (document.activeElement !== element) { return; }
         element.blur();
diff --git a/ext/js/dom/document-util.js b/ext/js/dom/document-util.js
index 73525ff5..ed0a98e7 100644
--- a/ext/js/dom/document-util.js
+++ b/ext/js/dom/document-util.js
@@ -21,6 +21,9 @@
  * TextSourceRange
  */
 
+/**
+ * This class contains utility functions related to the HTML document.
+ */
 class DocumentUtil {
     /**
      * Options to configure how element detection is performed.
@@ -251,11 +254,23 @@ class DocumentUtil {
         return scale;
     }
 
+    /**
+     * Converts a rect based on the CSS zoom scaling for a given node.
+     * @param {DOMRect} rect The rect to convert.
+     * @param {Node} node The node to compute the zoom from.
+     * @returns {DOMRect} The updated rect, or the same rect if no change is needed.
+     */
     static convertRectZoomCoordinates(rect, node) {
         const scale = this.computeZoomScale(node);
         return (scale === 1 ? rect : new DOMRect(rect.left * scale, rect.top * scale, rect.width * scale, rect.height * scale));
     }
 
+    /**
+     * Converts multiple rects based on the CSS zoom scaling for a given node.
+     * @param {DOMRect[]} rects The rects to convert.
+     * @param {Node} node The node to compute the zoom from.
+     * @returns {DOMRect[]} The updated rects, or the same rects array if no change is needed.
+     */
     static convertMultipleRectZoomCoordinates(rects, node) {
         const scale = this.computeZoomScale(node);
         if (scale === 1) { return rects; }
@@ -266,6 +281,13 @@ class DocumentUtil {
         return results;
     }
 
+    /**
+     * Checks whether a given point is contained within a rect.
+     * @param {number} x The horizontal coordinate.
+     * @param {number} y The vertical coordinate.
+     * @param {DOMRect} rect The rect to check.
+     * @returns {boolean} `true` if the point is inside the rect, `false` otherwise.
+     */
     static isPointInRect(x, y, rect) {
         return (
             x >= rect.left && x < rect.right &&
@@ -273,6 +295,13 @@ class DocumentUtil {
         );
     }
 
+    /**
+     * Checks whether a given point is contained within any rect in a list.
+     * @param {number} x The horizontal coordinate.
+     * @param {number} y The vertical coordinate.
+     * @param {DOMRect[]} rects The rect to check.
+     * @returns {boolean} `true` if the point is inside any of the rects, `false` otherwise.
+     */
     static isPointInAnyRect(x, y, rects) {
         for (const rect of rects) {
             if (this.isPointInRect(x, y, rect)) {
@@ -282,6 +311,13 @@ class DocumentUtil {
         return false;
     }
 
+    /**
+     * Checks whether a given point is contained within a selection range.
+     * @param {number} x The horizontal coordinate.
+     * @param {number} y The vertical coordinate.
+     * @param {Selection} selection The selection to check.
+     * @returns {boolean} `true` if the point is inside the selection, `false` otherwise.
+     */
     static isPointInSelection(x, y, selection) {
         for (let i = 0; i < selection.rangeCount; ++i) {
             const range = selection.getRangeAt(i);
@@ -302,6 +338,11 @@ class DocumentUtil {
         }
     }
 
+    /**
+     * Gets an array of the active modifier keys.
+     * @param {KeyboardEvent|MouseEvent|TouchEvent} event The event to check.
+     * @returns {string[]} An array of modifiers.
+     */
     static getActiveModifiers(event) {
         const modifiers = [];
         if (event.altKey) { modifiers.push('alt'); }
@@ -311,18 +352,33 @@ class DocumentUtil {
         return modifiers;
     }
 
+    /**
+     * Gets an array of the active modifier keys and buttons.
+     * @param {KeyboardEvent|MouseEvent|TouchEvent} event The event to check.
+     * @returns {string[]} An array of modifiers and buttons.
+     */
     static getActiveModifiersAndButtons(event) {
         const modifiers = this.getActiveModifiers(event);
         this._getActiveButtons(event, modifiers);
         return modifiers;
     }
 
+    /**
+     * Gets an array of the active buttons.
+     * @param {KeyboardEvent|MouseEvent|TouchEvent} event The event to check.
+     * @returns {string[]} An array of modifiers and buttons.
+     */
     static getActiveButtons(event) {
         const buttons = [];
         this._getActiveButtons(event, buttons);
         return buttons;
     }
 
+    /**
+     * Adds a fullscreen change event listener. This function handles all of the browser-specific variants.
+     * @param {Function} onFullscreenChanged The event callback.
+     * @param {?EventListenerCollection} eventListenerCollection An optional `EventListenerCollection` to add the registration to.
+     */
     static addFullscreenChangeEventListener(onFullscreenChanged, eventListenerCollection=null) {
         const target = document;
         const options = false;
@@ -341,6 +397,10 @@ class DocumentUtil {
         }
     }
 
+    /**
+     * Returns the current fullscreen element. This function handles all of the browser-specific variants.
+     * @returns {?Element} The current fullscreen element, or `null` if the window is not fullscreen.
+     */
     static getFullscreenElement() {
         return (
             document.fullscreenElement ||
@@ -351,6 +411,11 @@ class DocumentUtil {
         );
     }
 
+    /**
+     * Gets all of the nodes within a `Range`.
+     * @param {Range} range The range to check.
+     * @returns {Node[]} The list of nodes.
+     */
     static getNodesInRange(range) {
         const end = range.endContainer;
         const nodes = [];
@@ -361,6 +426,11 @@ class DocumentUtil {
         return nodes;
     }
 
+    /**
+     * Gets the next node after a specified node. This traverses the DOM in its logical order.
+     * @param {Node} node The node to start at.
+     * @returns {?Node} The next node, or `null` if there is no next node.
+     */
     static getNextNode(node) {
         let next = node.firstChild;
         if (next === null) {
@@ -377,6 +447,12 @@ class DocumentUtil {
         return next;
     }
 
+    /**
+     * Checks whether any node in a list of nodes matches a selector.
+     * @param {Node[]} nodes The list of ndoes to check.
+     * @param {string} selector The selector to test.
+     * @returns {boolean} `true` if any element node matches the selector, `false` otherwise.
+     */
     static anyNodeMatchesSelector(nodes, selector) {
         const ELEMENT_NODE = Node.ELEMENT_NODE;
         for (let node of nodes) {
@@ -389,6 +465,12 @@ class DocumentUtil {
         return false;
     }
 
+    /**
+     * Checks whether every node in a list of nodes matches a selector.
+     * @param {Node[]} nodes The list of ndoes to check.
+     * @param {string} selector The selector to test.
+     * @returns {boolean} `true` if every element node matches the selector, `false` otherwise.
+     */
     static everyNodeMatchesSelector(nodes, selector) {
         const ELEMENT_NODE = Node.ELEMENT_NODE;
         for (let node of nodes) {
@@ -401,10 +483,20 @@ class DocumentUtil {
         return true;
     }
 
+    /**
+     * Checks whether the meta key is supported in the browser on the specified operating system.
+     * @param {string} os The operating system to check.
+     * @param {string} browser The browser to check.
+     * @returns {boolean} `true` if supported, `false` otherwise.
+     */
     static isMetaKeySupported(os, browser) {
         return !(browser === 'firefox' || browser === 'firefox-mobile') || os === 'mac';
     }
 
+    /**
+     * Checks whether an element on the page that can accept input is focused.
+     * @returns {boolean} `true` if an input element is focused, `false` otherwise.
+     */
     static isInputElementFocused() {
         const element = document.activeElement;
         if (element === null) { return false; }
diff --git a/ext/js/dom/text-source-element.js b/ext/js/dom/text-source-element.js
index 13c5cd86..1c60a5b8 100644
--- a/ext/js/dom/text-source-element.js
+++ b/ext/js/dom/text-source-element.js
@@ -20,7 +20,18 @@
  * StringUtil
  */
 
+/**
+ * This class represents a text source that is attached to a HTML element, such as an <img>
+ * with alt text or a <button>.
+ */
 class TextSourceElement {
+    /**
+     * Creates a new instance of the class.
+     * @param {Element} element The source element.
+     * @param {string} fullContent The string representing the element's full text value.
+     * @param {number} startOffset The text start offset position within the full content.
+     * @param {number} endOffset The text end offset position within the full content.
+     */
     constructor(element, fullContent, startOffset, endOffset) {
         this._element = element;
         this._fullContent = fullContent;
@@ -29,38 +40,76 @@ class TextSourceElement {
         this._content = this._fullContent.substring(this._startOffset, this._endOffset);
     }
 
+    /**
+     * Gets the type name of this instance.
+     * @type {string}
+     */
     get type() {
         return 'element';
     }
 
+    /**
+     * The source element.
+     * @type {Element}
+     */
     get element() {
         return this._element;
     }
 
+    /**
+     * The string representing the element's full text value.
+     * @type {string}
+     */
     get fullContent() {
         return this._fullContent;
     }
 
+    /**
+     * The text start offset position within the full content.
+     * @type {number}
+     */
     get startOffset() {
         return this._startOffset;
     }
 
+    /**
+     * The text end offset position within the full content.
+     * @type {number}
+     */
     get endOffset() {
         return this._endOffset;
     }
 
+    /**
+     * Creates a clone of the instance.
+     * @returns {TextSourceElement} The new clone.
+     */
     clone() {
         return new TextSourceElement(this._element, this._fullContent, this._startOffset, this._endOffset);
     }
 
+    /**
+     * Performs any cleanup that is necessary after the element has been used.
+     */
     cleanup() {
         // NOP
     }
 
+    /**
+     * Gets the selected text of element, which is a substring of the full content
+     * starting at `startOffset` and ending at `endOffset`.
+     * @returns {string} The text content.
+     */
     text() {
         return this._content;
     }
 
+    /**
+     * Moves the end offset of the text by a set amount of unicode codepoints.
+     * @param {number} length The maximum number of codepoints to move by.
+     * @param {boolean} fromEnd Whether to move the offset from the current end position (if `true`) or the start position (if `false`).
+     * @returns {number} The actual number of characters (not codepoints) that were read.
+     */
     setEndOffset(length, fromEnd) {
         const offset = fromEnd ? this._endOffset : this._startOffset;
         length = Math.min(this._fullContent.length - offset, length);
@@ -72,6 +121,11 @@ class TextSourceElement {
         return length;
     }
 
+    /**
+     * Moves the start offset of the text by a set amount of unicode codepoints.
+     * @param {number} length The maximum number of codepoints to move by.
+     * @returns {number} The actual number of characters (not codepoints) that were read.
+     */
     setStartOffset(length) {
         length = Math.min(this._startOffset, length);
         if (length > 0) {
@@ -82,22 +136,42 @@ class TextSourceElement {
         return length;
     }
 
+    /**
+     * Gets the rects that represent the position and bounds of the text source.
+     * @returns {DOMRect[]} The rects.
+     */
     getRects() {
         return DocumentUtil.convertMultipleRectZoomCoordinates(this._element.getClientRects(), this._element);
     }
 
+    /**
+     * Gets writing mode that is used for this element.
+     * See: https://developer.mozilla.org/en-US/docs/Web/CSS/writing-mode.
+     * @returns {string} The rects.
+     */
     getWritingMode() {
         return 'horizontal-tb';
     }
 
+    /**
+     * Selects the text source in the document.
+     */
     select() {
         // NOP
     }
 
+    /**
+     * Deselects the text source in the document.
+     */
     deselect() {
         // NOP
     }
 
+    /**
+     * Checks whether another text source has the same starting point.
+     * @param {TextSourceElement|TextSourceRange} other The other source to test.
+     * @returns {boolean} `true` if the starting points are equivalent, `false` otherwise.
+     */
     hasSameStart(other) {
         return (
             typeof other === 'object' &&
@@ -109,14 +183,28 @@ class TextSourceElement {
         );
     }
 
+    /**
+     * Gets a list of the nodes in this text source's range.
+     * @returns {Node[]} The nodes in the range.
+     */
     getNodesInRange() {
         return [this._element];
     }
 
+    /**
+     * Creates a new instance for a given element.
+     * @param {Element} element The source element.
+     * @returns {TextSourceElement} A new instance of the class corresponding to the element.
+     */
     static create(element) {
         return new TextSourceElement(element, this._getElementContent(element), 0, 0);
     }
 
+    /**
+     * Gets the full content string for a given element.
+     * @param {Element} element The element to get the full content of.
+     * @returns {string} The content string.
+     */
     static _getElementContent(element) {
         let content;
         switch (element.nodeName.toUpperCase()) {
diff --git a/ext/js/dom/text-source-range.js b/ext/js/dom/text-source-range.js
index e03783a5..a0225748 100644
--- a/ext/js/dom/text-source-range.js
+++ b/ext/js/dom/text-source-range.js
@@ -20,7 +20,29 @@
  * DocumentUtil
  */
 
+/**
+ * This class represents a text source that comes from text nodes in the document.
+ * Sometimes a temporary "imposter" element is created and used to store the text.
+ * This element is typically hidden from the page and removed after scanning has completed.
+ */
 class TextSourceRange {
+    /**
+     * Creates a new instance of the class.
+     * @param {Range} range The selection range.
+     * @param {number} rangeStartOffset The `startOffset` of the range. This is somewhat redundant
+     *   with the `range` parameter, but it is used when for when imposter elements are removed.
+     * @param {string} content The `toString()` value of the range. This is somewhat redundant
+     *   with the `range` parameter, but it is used when for when imposter elements are removed.
+     * @param {?Element} imposterElement The temporary imposter element.
+     * @param {?Element} imposterSourceElement The source element which the imposter is imitating.
+     *   Must not be `null` if imposterElement is specified.
+     * @param {?Rect[]} cachedRects A set of cached `DOMRect`s representing the rects of the text source,
+     *   which can be used after the imposter element is removed from the page.
+     *   Must not be `null` if imposterElement is specified.
+     * @param {?Rect} cachedSourceRect A cached `DOMRect` representing the rect of the `imposterSourceElement`,
+     *   which can be used after the imposter element is removed from the page.
+     *   Must not be `null` if imposterElement is specified.
+     */
     constructor(range, rangeStartOffset, content, imposterElement, imposterSourceElement, cachedRects, cachedSourceRect) {
         this._range = range;
         this._rangeStartOffset = rangeStartOffset;
@@ -31,22 +53,42 @@ class TextSourceRange {
         this._cachedSourceRect = cachedSourceRect;
     }
 
+    /**
+     * Gets the type name of this instance.
+     * @type {string}
+     */
     get type() {
         return 'range';
     }
 
+    /**
+     * The internal range object.
+     * @type {Range}
+     */
     get range() {
         return this._range;
     }
 
+    /**
+     * The starting offset for the range.
+     * @type {Range}
+     */
     get rangeStartOffset() {
         return this._rangeStartOffset;
     }
 
+    /**
+     * The source element that the imposter element is imitating, if present.
+     * @type {?Element}
+     */
     get imposterSourceElement() {
         return this._imposterSourceElement;
     }
 
+    /**
+     * Creates a clone of the instance.
+     * @returns {TextSourceRange} The new clone.
+     */
     clone() {
         return new TextSourceRange(
             this._range.cloneRange(),
@@ -59,16 +101,31 @@ class TextSourceRange {
         );
     }
 
+    /**
+     * Performs any cleanup that is necessary after the element has been used.
+     */
     cleanup() {
         if (this._imposterElement !== null && this._imposterElement.parentNode !== null) {
             this._imposterElement.parentNode.removeChild(this._imposterElement);
         }
     }
 
+    /**
+     * Gets the selected text of element, which is the `toString()` version of the range.
+     * @returns {string} The text content.
+     */
     text() {
         return this._content;
     }
 
+    /**
+     * Moves the end offset of the text by a set amount of unicode codepoints.
+     * @param {number} length The maximum number of codepoints to move by.
+     * @param {boolean} fromEnd Whether to move the offset from the current end position (if `true`) or the start position (if `false`).
+     * @param {boolean} layoutAwareScan Whether or not HTML layout information should be used to generate
+     *   the string content when scanning.
+     * @returns {number} The actual number of characters (not codepoints) that were read.
+     */
     setEndOffset(length, fromEnd, layoutAwareScan) {
         let node;
         let offset;
@@ -85,6 +142,13 @@ class TextSourceRange {
         return length - state.remainder;
     }
 
+    /**
+     * Moves the start offset of the text by a set amount of unicode codepoints.
+     * @param {number} length The maximum number of codepoints to move by.
+     * @param {boolean} layoutAwareScan Whether or not HTML layout information should be used to generate
+     *   the string content when scanning.
+     * @returns {number} The actual number of characters (not codepoints) that were read.
+     */
     setStartOffset(length, layoutAwareScan) {
         const state = new DOMTextScanner(this._range.startContainer, this._range.startOffset, !layoutAwareScan, layoutAwareScan).seek(-length);
         this._range.setStart(state.node, state.offset);
@@ -93,16 +157,28 @@ class TextSourceRange {
         return length - state.remainder;
     }
 
+    /**
+     * Gets the rects that represent the position and bounds of the text source.
+     * @returns {DOMRect[]} The rects.
+     */
     getRects() {
         if (this._isImposterDisconnected()) { return this._getCachedRects(); }
         return DocumentUtil.convertMultipleRectZoomCoordinates(this._range.getClientRects(), this._range.startContainer);
     }
 
+    /**
+     * Gets writing mode that is used for this element.
+     * See: https://developer.mozilla.org/en-US/docs/Web/CSS/writing-mode.
+     * @returns {string} The rects.
+     */
     getWritingMode() {
         const node = this._isImposterDisconnected() ? this._imposterSourceElement : this._range.startContainer;
         return DocumentUtil.getElementWritingMode(node !== null ? node.parentElement : null);
     }
 
+    /**
+     * Selects the text source in the document.
+     */
     select() {
         if (this._imposterElement !== null) { return; }
         const selection = window.getSelection();
@@ -110,12 +186,22 @@ class TextSourceRange {
         selection.addRange(this._range);
     }
 
+    /**
+     * Deselects the text source in the document.
+     */
     deselect() {
         if (this._imposterElement !== null) { return; }
         const selection = window.getSelection();
         selection.removeAllRanges();
     }
 
+    /**
+     * Checks whether another text source has the same starting point.
+     * @param {TextSourceElement|TextSourceRange} other The other source to test.
+     * @returns {boolean} `true` if the starting points are equivalent, `false` otherwise.
+     * @throws {Error} An exception can be thrown if `Range.compareBoundaryPoints` fails,
+     *   which shouldn't happen, but the handler is kept in case of unexpected errors.
+     */
     hasSameStart(other) {
         if (!(
             typeof other === 'object' &&
@@ -142,24 +228,48 @@ class TextSourceRange {
         }
     }
 
+    /**
+     * Gets a list of the nodes in this text source's range.
+     * @returns {Node[]} The nodes in the range.
+     */
     getNodesInRange() {
         return DocumentUtil.getNodesInRange(this._range);
     }
 
+    /**
+     * Creates a new instance for a given range.
+     * @param {Range} range The source range.
+     * @returns {TextSourceRange} A new instance of the class corresponding to the range.
+     */
     static create(range) {
         return new TextSourceRange(range, range.startOffset, range.toString(), null, null, null, null);
     }
 
+    /**
+     * Creates a new instance for a given range using an imposter element.
+     * @param {Range} range The source range.
+     * @param {Element} imposterElement The temporary imposter element.
+     * @param {Element} imposterSourceElement The source element which the imposter is imitating.
+     * @returns {TextSourceRange} A new instance of the class corresponding to the range.
+     */
     static createFromImposter(range, imposterElement, imposterSourceElement) {
         const cachedRects = DocumentUtil.convertMultipleRectZoomCoordinates(range.getClientRects(), range.startContainer);
         const cachedSourceRect = DocumentUtil.convertRectZoomCoordinates(imposterSourceElement.getBoundingClientRect(), imposterSourceElement);
         return new TextSourceRange(range, range.startOffset, range.toString(), imposterElement, imposterSourceElement, cachedRects, cachedSourceRect);
     }
 
+    /**
+     * Checks whether the imposter element has been removed, if the instance is using one.
+     * @returns {boolean} `true` if the instance has an imposter and it's no longer connected to the document, `false` otherwise.
+     */
     _isImposterDisconnected() {
         return this._imposterElement !== null && !this._imposterElement.isConnected;
     }
 
+    /**
+     * Gets the cached rects for a disconnected imposter element.
+     * @returns {Rect[]} The rects for the element.
+     */
     _getCachedRects() {
         const sourceRect = DocumentUtil.convertRectZoomCoordinates(this._imposterSourceElement.getBoundingClientRect(), this._imposterSourceElement);
         return DocumentUtil.offsetDOMRects(