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Round Trip Time (RTT) is the length time it takes for a data packet to be sent to a destination plus the time it takes for an acknowledgment of that packet to be received back at the origin. The RTT between a network and server can be determined by using the ping command.

The MDN Web Docs site provides information about Open Web technologies including HTML, CSS, and APIs for both Web sites and progressive web apps.

Events are fired to notify code of "interesting changes" that may affect code execution. These can arise from user interactions such as using a mouse or resizing a window, changes in the state of the underlying environment (e.g. low battery or media events from the operating system), and other causes.

The CSS Object Model (CSSOM) is a set of APIs for reading and modifying a document's style-related (CSS) information. In other words, similar to the way in which the DOM enables a document's structure and content to be read and modified from JavaScript, the CSSOM allows the document's styling to be read and modified from JavaScript.

Brotli is a general-purpose lossless compression algorithm.

Time to Interactive (TTI) is a non-standardized web performance 'progress' metric defined as the point in time when the last Long Task finished and was followed by 5 seconds of network and main thread inactivity.

First Contentful Paint (FCP) is when the browser renders the first bit of content from the DOM, providing the first feedback to the user that the page is actually loading. The question "Is it happening?" is "yes" when the first contentful paint completes.

First input delay (FID) measures the time from when a user first interacts with your site (i.e. when they click a link, tap on a button, or use a custom, JavaScript-powered control) to the time when the browser is actually able to respond to that interaction.

Code splitting is the splitting of code into various bundles or components which can then be loaded on demand or in parallel.

Browsers limit the number of active connections for each domain. To enable concurrent downloads of assets exceeding that limit, domain sharding splits content across multiple subdomains. When multiple domains are used to serve multiple assets, browsers are able to download more resources simultaneously, resulting in a faster page load time and improved user experience.

A web beacon is a small object, such as a 1 pixel gif, embedded in markup, used to communicate information back to the web server or to 3rd party servers. Beacons are generally included to provide information about the user for statistical purposes. Beacons are often included within third party scripts for collecting user data, performance metrics and error reporting.

First Interactive, also known as first CPU idle, is a non-standard web performance metric that measures when the user's window is quiet enough to handle user input, or what is termed as minimally interactive.

First CPU Idle measures when a page is minimally interactive, or when the window is quiet enough to handle user input. It is a non-standard Google web performance metric. Generally, it occurs when most, but not necessarily all visible UI elements are interactive, and the user interface responds, on average, to most user input within 50ms. It is also known as First interactive.

Effective connection type (ECT) refers to the measured network performance, returning a cellular connection type, like 3G, even if the actual connection is tethered broadband or Wi-Fi, based on the time between the browser requesting a page and effective type of the connection.

Jank refers to sluggishness in a user interface, usually caused by executing long tasks on the main thread, blocking rendering, or expending too much processor power on background processes.

First Paint, part of the Paint Timing API, is the time between navigation and when the browser renders the first pixels to the screen, rendering anything that is visually different from the default background color of the body. It answers the question "Is it happening?"

First Meaningful Paint (FMP) is the paint after which the biggest above-the-fold layout change has happened and web fonts have loaded. It is when the answer to "Is it useful?" becomes "yes", upon first meaningful paint completion.

Lossy compression, or irreversible compression, is a data-compression method that uses inexact approximations and partial-data discarding to represent content. In simpler terms: lossy compression causes data from the initial file to be lost, possibly causing degradation in quality. The process of such compression is irreversible; once lossy compression of the content has been performed, the content cannot be restored to its original state. Therefore, content that has undergone lossy compression shoul

Lazy loading is a strategy that delays the loading of some assets (e.g., images) until they are needed by the user based on the user's activity and navigation pattern; typically, these assets are only loaded when they are scrolled into view.

Building websites requires HTML, CSS, and JavaScript. To build websites and applications people want to use, which attract and retain users, you need to create a good user experience. Part of good user experience is ensuring the content is quick to load and responsive to user interaction. This is known as web performance, and in this module you'll focus on the fundamentals of how to create performant websites.

HTTP/2 is a major revision of the HTTP network protocol.

Minification is the process of removing unnecessary or redundant data without affecting how a resource is processed by the browser.

Lossless compression is a class of data compression algorithms that allows the original data to be perfectly reconstructed from the compressed data. Lossless compression methods are reversible. Examples of lossless compression include GZIP, Brotli, WebP, and PNG,

A long task is a task that takes more than 50ms to complete.

Latency is the network time it takes for a requested resource to reach its destination. Low latency is good, meaning there is little or no delay. High latency is bad, meaning it takes a long time for the requested resource to reach its destination.