The navigator.onLine property is a boolean for whether the browser is connected to some network (though not necessarily the internet). The online and offline events fire when the connection state changes.

The line-height CSS property sets the spacing between text baselines, oriented to the horizontal or vertical writing mode.

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 display: list-item CSS declaration renders an element with the box layout of a <li> HTML element.

The document.evaluate() method selects elements in an HTML or XML document based on an expression of XPath, a domain specific language for querying XML documents. Also known as XML Path Language.

The letter-spacing CSS property controls the amount of space between each letter in an element or block of text.

The text-align CSS property sets the horizontal placement of the inner content of a block element.

The text-transform CSS property sets text case and capitalization.

The word-spacing CSS property sets the amount of white space between words.

The text-decoration CSS property sets the style and color of decorative lines including underline, overline, line-through, or a combination of lines.

The @namespace CSS rule sets a default namespace or namespace prefix. Namespace prefixes allow CSS selectors to distinguish elements with the same name but different document types, such as the HTML <a> element and the SVG <a> element.

The localStorage and sessionStorage APIs store data as key-value pairs. While localStorage persists across sessions, sessionStorage data is discarded when the page is closed.

The <filter> SVG element applies custom effects such as color manipulation, blurring, or morphing to SVG elements.

The cubic-bezier() CSS easing function interpolates along a smooth curve, creating animations and transitions with continuous changes in speed. The ease, ease-in, ease-out, and ease-in-out keyword values are presets for common Bézier curves.

The visibility CSS property sets whether an element is shown. Invisible elements still affect the document layout.

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 ch CSS length unit is a font-relative length based on the width of the zero (0) character.

The em CSS length unit is a font-relative length equal to the specified font size. In an element with a 2 inch font, 1em equals 2 inches.

The ex CSS length unit is a font-relative length equal to the used x-height of the first available font. X-height is often equal to the height of the lowercase x.

The rem CSS length unit is a font-relative length that is equal to the font size of the root HTML element which for most browsers is 16px.

A URL object represents a web address, like https://example.com/, and parts of the address, such as the domain, path, and query string (as URLSearchParams).

The postMessage() global method sends cross-origin messages to windows and workers, including popups and iframes. Also known as cross-document messaging.

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 location global object represents the current page's address. You can use it to get the parts of the address (such as location.hostname or location.pathname) or navigate to another URL.

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

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

The console API logs debugging messages to the browser development tools' console.

The hashchange event fires when the URL fragment identifier (the part of the URL starting with #) of the current page has changed.

The min-width, min-height, max-width, and max-height CSS properties set the minimum and maximum size of an element.

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.