The counter-reset and counter-increment CSS properties and the counter() and counters() functions automatically number headings or ordered list items.

The DOM (Document Object Model) API represents HTML and XML documents as trees of nodes. You can use the API to inspect and modify the structure and content of a document.

Data URLs are URLs that start with the data: scheme and which directly contain data rather than point to remote resources.

The dataset API attaches and accesses custom data on elements, using either the dataset property of HTMLElement or attributes prefixed with data-.

The Date object represents a single moment in time.

The Worker() constructor runs a script in its own thread, which can send and receive messages with the script that created it. Also known as web workers.

The <dl>, <dt>, and <dd> HTML elements represent a list of terms and their associated descriptions. Use a description list to create glossaries or similar list of key-value pairs.

The display CSS property sets the display behavior of an element's box within its layout and sets the layout behavior for its child elements.

The Drag and Drop API lets users drag and drop elements and external files such as images onto web pages. Developers can customize which elements can become draggable, the type of feedback the draggable elements produce, and the droppable elements.

The <input type="email">, <input type="tel">, and <input type="url"> HTML elements represent email address, telephone number, and URL fields in a form.

The Event API and the addEventListener() method on objects that receive events (event targets) represent and handle significant things happening on a page. Many APIs fire events for a wide range of situations relating to those APIs, such as an image loading, a user clicking, or a value changing.

The position: fixed CSS declaration removes an element from the normal flow and positions it relative to the viewport or page.

Focus events, such as focus and blur, fire when an element gets or loses focus.

The font CSS property shorthand sets multiple font properties, including style, weight, size, and font family.

The form validity CSS pseudo-classes match <form> elements based on the constraints of a form field, such as validity (:valid, :invalid, :in-range, :out-of-range) and necessity (:optional or :required).

Functions are series of statements that can be called and return a value. The function keyword (as in function () { }) and arrow (=>) expression create functions. The JavaScript functions protocol includes default and rest parameters and binding to this.

The GIF (Graphics Interchange Format) image format is a raster graphics file format with animation and lossless compression.

The navigator.geolocation API requests the user's latitude, longitude, heading, and speed.

The linear-gradient() and radial-gradient() CSS functions and their repeating counterparts create backgrounds that progress smoothly between multiple colors.

The Strict-Transport-Security HTTP response header informs browsers that the site should only be accessed using HTTPS, and that any future attempts to access it using HTTP should automatically be converted to HTTPS. Also known as HSTS.

The Authorization and WWW-Authenticate headers authenticate a user with a server.

HTTP/1.1 is a network protocol used by browsers and servers. It has been superseded by HTTP/2 and HTTP/3.

The window.history API manipulates the browser session history, from navigations to state management, in the tab or frame that the current page is loaded in.

The <area> and <map> elements define a clickable area on an image.

The :checked, :disabled, and :enabled CSS pseudo-classes match form elements based on their state.

The for...of loop operates on a sequence of values sourced from an iterable object, such as arrays, array-like objects, DOM collections, iterators, generators, and user-defined iterables. All built-in iterators inherit from the Iterator class.

The JPEG image format is a raster graphics file format with lossy compression.

The JSON API provides static methods for parsing values from and converting values to JavaScript Object Notation (JSON), a serialization format for objects, arrays, numbers, strings, Boolean values, and null.

JavaScript is a programming language that runs in browsers, usually through the <script> element. JavaScript has changed over many years. This feature represents the oldest language features, such as built-in objects, statements, and operators. Also known as ECMAScript.

The keydown and keyup events fire for each key press (or, with modifier keys, a combination of key presses) on a keyboard.