HTML Glossary: 46 Web Terms Explained Simply

Q: What does nesting mean in HTML?

Putting one element inside another, such as an <li> inside a <ul> . Close the inner element before the outer one: Hi is correct, and closing the paragraph first is not. Indent nested code and the structure becomes much easier to follow. A WYSIWYG HTML editor that shows formatted text next to its source is a handy way to watch nesting happen.

HTML comes with a lot of new words. This glossary defines 46 of them in one or two plain sentences each, without jargon. It covers the basics, such as element, tag, attribute and nesting, and the terms you meet later on, such as DOM, landmark, entity and viewport. Lessons link here the first time they use a term, so you can look up a meaning without losing your place. When a definition is not enough, open the related lesson or the tag reference. The official definitions are in the HTML Living Standard (opens in a new tab), but those are written for the people who build browsers, and it shows.

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absolute path: A complete address with the protocol and domain, such as https://example.com/images/cat.png. It leads to the same file from any page that uses it.
accessibility: Making pages that everyone can use, including people who are blind or deaf, who use only a keyboard, or who have other disabilities. It starts with well-structured HTML.
alt text: The description of an image, written in its alt attribute. Screen readers read it out, and the browser shows it when the image fails to load.
anchor: The a element, which makes a link. The word is also used for a link target: the spot on a page that a #id link jumps to.
aria: Accessible Rich Internet Applications: the role attribute plus the aria-* attributes, which pass extra information to assistive technology. Use it only when no native HTML element can do the job.
attribute: A name="value" pair written inside an opening tag, such as href on a link or alt on an image. It sets how the element behaves or what it points to.
block: An element that starts on a new line and, by default, fills the full available width, such as p, h1 or div. Block elements stack one under another.
body: The element that holds everything visible on a page. It sits inside the html element, right after the head.
box model: The CSS rule that every element is a rectangle built from content, padding, border and margin, from the inside out. It decides how big an element is and how much space it takes.
browser: A program such as Chrome, Firefox, Safari or Edge that downloads web pages and shows them. It reads your HTML and draws the result on screen.
caption: A title or short description attached to some content. In HTML, caption names a table and figcaption describes a figure.
cell: One box in a table, the spot where a row crosses a column. Data cells use td; header cells use th.
character encoding: The system that turns characters into the bytes stored in a file. Use UTF-8, declared with <meta charset="utf-8">, and every language and symbol will display correctly.
comment: A note in the source code between . The browser skips it, so visitors never see it on the page.
css: Cascading Style Sheets: the language that sets how HTML looks, from colors and fonts to spacing and layout. HTML describes the structure and CSS handles the style.
doctype: The line <!doctype html> at the very top of a page. It switches the browser into modern standards mode instead of an old compatibility mode.
dom: The Document Object Model: the tree of elements the browser builds in memory from your HTML. CSS and JavaScript work on this tree, and it can differ from your source when the browser has quietly fixed mistakes.
element: One piece of a page, made of an opening tag, content and a closing tag, such as Hello. Every HTML document is built out of elements.
entity: A code that stands for a character, written between & and ;, such as < for < or © for the copyright sign. You need one for any character HTML would otherwise read as code.
favicon: The small icon in the browser tab and in bookmarks. You set it with a link element such as <link rel="icon" href="favicon.svg">.
focus: The state of the element that is taking keyboard input right now, such as a selected link or text field. Press Tab and focus moves to the next focusable element.
form control: An element where the user types or picks data, such as input, select, textarea or button. Form controls usually sit inside a form element.
head: The element that holds information about the page, such as its title, character encoding and stylesheets. Nothing inside it appears on the page itself.
html: HyperText Markup Language: the language that describes the structure and content of web pages. Its tags mark text as headings, paragraphs, links, images and more.
iframe: An inline frame: an element that displays another web page inside the current one. Embedded maps, videos and widgets usually arrive this way.
inline: An element that sits inside a line of text and is only as wide as its content, such as a, strong or span. It does not start a new line.
label: The visible name of a form control, written with the label element. Once it is tied to its control, clicking the label focuses the control, and screen readers announce it.
landmark: A major region of a page, such as header, nav, main, aside or footer. Screen reader users can jump straight from one landmark to the next.
metadata: Information about the page that is not shown on it, such as its title, description, language and character encoding. Most of it goes in the head.
nesting: Putting elements inside other elements, like a list inside a list item. Close the inner element before the outer one: Hi.
property: In CSS, the name of the feature you want to change, such as color or font-size. A colon and a value come after it: color: red.
relative path: An address written from the current page's position, such as about.html or images/cat.png. It keeps working as long as the files stay in the same places relative to each other.
screen reader: Software that reads what is on the screen aloud or sends it to a braille display. Blind and low-vision people use it to browse the web.
selector: The part of a CSS rule that picks which elements to style, such as p, .note or #menu. The automatic checker in this course uses selectors to find elements too.
semantic: Describing what content is, with no reference to how it looks. Semantic HTML uses elements named for their meaning, such as nav for navigation or button for a button.
seo: Search Engine Optimization: making pages easy for search engines to understand and rank. Good titles, descriptions and headings help, and so does semantic HTML.
source code: The text you write, such as the HTML in the editor, before the browser displays it. Every browser lets you open the source code of any page.
stylesheet: A set of CSS rules that style a page. You can put it in a style element or in a separate .css file loaded with a link element.
table: Data arranged in rows and columns, built with the table element. Use tables for data, never for page layout.
tag: A keyword in angle brackets that marks where an element starts or ends, such as or . A closing tag has a slash before the name.
url: Uniform Resource Locator: the address of something on the web, such as https://example.com/page.html. Links, images and scripts all point to URLs.
validation: Checking that data or code follows the rules. Browsers check form fields before sending (an empty required field, a badly formed email), and a validator checks that your HTML is correct.
viewport: The visible area of a page in the browser window or on a phone screen. The viewport meta tag tells phones to use their real width and stop shrinking a desktop layout.
void element: An element that cannot hold content and has no closing tag, such as img, br, input or meta. Everything it needs goes in its attributes.
whitespace: Spaces, tabs and line breaks in the source code. Browsers collapse any run of them into a single space, except inside pre.
wysiwyg: What You See Is What You Get: an editor that shows content as it will look while you edit, much like a word processor. A code editor shows the source instead.

Frequently asked questions

What is the difference between HTML, CSS and JavaScript?

HTML describes the content and what it means, CSS sets how it looks, and JavaScript adds behavior such as reacting to clicks or loading new data. Picture a house. HTML is the walls and rooms, CSS is the paint and furniture, and JavaScript is the wiring. The HTML vs CSS vs JavaScript guide builds one example in all three.

What does nesting mean in HTML?

Putting one element inside another, such as an <li> inside a <ul>. Close the inner element before the outer one: Hi is correct, and closing the paragraph first is not. Indent nested code and the structure becomes much easier to follow. A WYSIWYG HTML editor (opens in a new tab) that shows formatted text next to its source is a handy way to watch nesting happen.

What is an HTML entity?

A code that stands for a character, written between & and ;. HTML reserves a few characters, so you write < for < and & for &. Others, such as ©, save you hunting for symbols that are hard to type. Wikipedia has a complete list of character entity references (opens in a new tab).