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Web browsers can be quite confusing to understand, especially once you consider the breadth of all their features. As with all software engineering—indeed, all complex subjects—the best way to avoid confusion is to use consistent and clear names.

Key web terms

Accessibility: The ability of any person to access and use a web page, regardless of ability, or technology to achieve the same.

Browser chrome: The UI of a browser, such as a tab or URL bar, not including the web page the browser is displaying.

HTML: HyperText Markup Language, the XML-like format used to describe web pages.

Hyperlink: A reference from one web page to another.

HTTP: HyperText Transport Protocol, the network protocol for loading web pages.

HTTPS: A variant of HTTP that uses cryptography to provide network security.

Hypertext: A non-linear form of information comprised of multiple documents connected with contextual links.

JavaScript: The main programming language for web scripts.

URL: Uniform Resource Locator, the name used to refer uniquely to a web page.

Rendering engine: The part of a web browser concerned with drawing a web page to the screen and interacting with it. There are three main rendering engines actively maintained today: Chromium, WebKit, and Gecko.

Script: A piece of code that extends a web page with more functionality, usually written in JavaScript.

Web security: The ability to intentionally limit the behavior of web browsers, servers, or applications, usually to prevent unintentional harm. There are lots of different aspects of security: browser security (so the user’s computer isn’t harmed by their browser), web application security (so a web application can’t be harmed by its users), privacy (so a third party can’t harm a web user), and many others.

Web: Simplified name for the WWW.

Web browser: A software program that allows people to load and navigate web pages. Also often just called a “browser”.

Web page: The basic unit of the web; defined by unique URL.

Web resource: Anything with its own URL on the web. Web pages are resources, but so are many of their component parts, such as scripts, images, and style sheets.

Website: A collection of web pages that together provide some user service.

WWW: World Wide Web. A name for the network of web pages built on HTTP, hyperlinks, HTML, CSS and JavaScript, as well as the open and decentralized rules that (informally) govern them.


IETF: Internet Engineering Task Force. The standardization organization for HTTP as well as some other APIs.

TC39: Technical Committee 39. The standardization organization for JavaScript.

Khronos: The Khronos Group. The standardization organization for WebGL and WebGPU.

W3C: World Wide Web Consortium. The central standardization organization of the WWW. Among many other APIs, this is where CSS is standardized.

WHATWG: Web Hypertext Application Technology Working Group. The standardization organization for HTML, DOM, and a few other key web APIs.

Web Documents

Animation: A sequence of visual changes on a computer screen interpreted by humans to look like movement.

HTML attribute: A parameter on an element indicating some information, such as the source of an image or URL of a style sheet.

Parsing: Turning a serialized representation (such as HTML or CSS) into a data structure such as the document tree or a style sheet.

CSS: Cascading Style Sheet. A format for representing rules that specify the (mostly visual) styling of elements in the DOM.

Document: The conceptual object created when loading a web page and modified by interacting with it, an analogy to physical documents.

Document tree: The tree created from parsing HTML. Also sometimes called the DOM.

DOM: Document Object Model. The object-oriented API interface to JavaScript for mutating the document. It contains in particular a tree of nodes; on first page load this tree corresponds to the nested structure of the HTML.

Element: Most nodes in the DOM tree are elements (except for text and the document object).

Event: A way for JavaScript to observe that something has happened on the document, and customize its results.

Focus: The property of an element (sometimes in the web page, sometimes in the browser chrome) being set to receive future keyboard events and other user interactions. Typically, the focused element is visually highlighted on the screen.

Font: A particular stylistic way of drawing a particular human language to computer screens. Times New Roman is one common example for Latin-based languages.

Iframe: A way of embedding one document within another. A rectangular window in the parent document shows the child document and participates in the layout of the parent.

Image: A representation of a picture to draw on a computer screen. An HTML element of the same name, for the same purpose.

Node: A point in the DOM tree, with parent and child pointers.

Page: The conceptual container for a document. A page can have multiple documents through use of iframes.

Style sheet: A web resource that contains CSS rules.

Tag name: The name of a particular type of HTML element, indicating its semantic function in the document. Usually comes with special style rules and functionality specific to it.


GET: The mode of HTTP that retrieves a server resource without changing it.

Cookie: A piece of persistent, per-site state stored by web browsers to enable use cases like user logged-in status for access-controlled content.

Domain: The name of a website, used to locate it on the internet.

Path: The part of a URL after the domain and port.

Port: A number after the domain and before the path in a URL, indicating a numbered place on that domain with which to communicate.

POST: The mode of HTTP that submits a change to server state and expects a newly updated web page in response.

Scheme: The first part of a URL, indicating which protocol to use for communication, such as HTTP or HTTPS.

TLS/SSL: Transport Layer Security and Secure Sockets Layer. An encrypted protocol atop which other protocols like HTTP can take place securely (i.e., HTTPS). TLS is a newer protocol replacing SSL, but SSL is often used to describe both.


Cascade order: The order of application of multiple CSS rules to a single element.

Computed style: The values for the CSS Properties that apply to elements after applying all rules according to the cascade order.

CSS property: A single concept (such as “color” or “width”) used to style a specific part of an element.

CSS property value: a key-value pair of a CSS property and its value (e.g. “color” and “blue” or “width” and “30px”).

CSS rule: The combination of a selector and property values.

CSS selector: The part of a CSS rule that specifies which elements a given list of property values applies.

Inheritance: When an element takes its computed style for a property from its parent element. Sometimes mistakenly called “cascading”. Some CSS properties (such as font sizing) are inherited by default.

Style: All the pieces of information necessary to determine the visual display of an element.

Cascade: The order in which to apply multiple rules to the same element.

Coordinate spaces

In the browser, 2D coordinate spaces are used to determine where is elements are relative to one another, the web page, and the screen. Most of these coordinate systems use the standard x and y directions but with different origins, though not all.Some logical coordinate systems flip the direction of x and y according to the direction of the writing mode of the language. For example, in Arabic it makes sense for x to grow towards the left, and the origin is often at the top-right, not the top-left. This becomes confusing when nesting, containing blocks, scrolling, and positioning are used together.

Viewport: This coordinate system’s origin is at the top-left of the rectangle on the screen into which the web page is drawn.

Page: This coordinate system’s origin is at the top-left of the web page’s root element. When a web page is scrolled, this top-left may be off the top of the viewport.

Element: This coordinate system’s origin is at the top-left of the layout bounds of the element, which may be off the top or left of the viewport if margins or positioning is used.

Paint: This coordinate system’s origin is at the top-left of the paint bounds of the element, which may not match the element coordinate system if transforms like translate are used. Real browsers also support more complex transforms such as rotate.

Layout: This coordinate system’s origin is at the top-left of a composited layer, which is chosen so as to include all of the paint objects within the layer.


Accessibility tree: A tree representing the semantic meaning of a web page meant for consumption by assistive technologies.

Canvas: A conceptual object which can execute drawing commands. Also a web API of the same name that serves the same purpose. Typically backed by a surface.

Compositing: The phase of a browser rendering pipeline that divides the display list into pieces suitable for rendering into independent surfaces on a GPU, in order to speed up animations.

Decode: converting from a compressed format for a resource (such as an an image) into a simpler format in memory (such as a bitmap).

Display list: A sequence of graphics commands explaining how to draw a web page to a computer screen.

Draw: The phase of a browser rendering pipeline that puts a set of surfaces onto the screen with various positions and visual effects.

Event loop: A loop that alternates between receiving user input and drawing to the screen.

Hit testing: Determining which element or accessibility tree node is at a given pixel location on the screen.

Invalidation: Marking some rendering state as no longer valid, because its input dependencies have changed.

Layout: The phase of a browser rendering pipeline that determines the size and position of elements in the DOM.

Layout tree: A second tree that mirrors the DOM, except that it represents the output of the layout pipeline phase.

Paint: The phase of a browser rendering pipeline that creates a display list from the DOM.

Rendering pipeline: The sequence of phases by which a browser draws a web page onto a computer screen.

Raster: The process of executing a display list by coloring the pixels of a surface.

Scroll: adjusting the horizontal or vertical offset of a web page in response to user input, in order to see parts of it not currently visible.

Style: The phase of a browser rendering pipeline that applies CSS rules to determine the visual appearance and behavior of elements in the DOM. Or, the set of CSS properties applied to an element after this phase.

Surface: A buffer or texture within a GPU that represents a 2D array of pixels.

Visual effect: A CSS property that does not affect layout.

Zoom: Changing the ratio of CSS sizes to pixels in order to make content on a web page larger or smaller.

Computer technologies

Assistive technology: Computer software used to assist people in using the computer or web browser. The most common are screen readers.

CPU: Central Processing Unit, the hardware component in a computer that executes generic compute programs.

DukPy: A JavaScript interpreter used in this book.

GPU: Graphics Processing Unit, a specialized computing chip optimized for tasks like generating pixel output on computer screens.

Process: A conceptual execution environment with its own code and memory, isolated from other processes by hardware and software computer mechanisms.

Python: A common computer programming language, used in this book to implement a toy browser.

Thread: A single sequence of commands executed on a CPU. Most CPUs have these days can execute multiple threads at once.

SDL: A windowing library for computer programs used in later chapters of this book.

Skia: A raster drawing library for computer programs used in later chapters of this book.

Tk: A UI drawing library for computer programs used in early chapters of this book.