A parameter or setting for an element, which can be set on a start tag.

For example, in the tag <a href="glossary.html">, the href attribute is set to “glossary.html”.

Most attributes are optional, but a few are required.

See Web browser.
Cascading Style Sheets. A style sheet language used to describe styles for HTML (and other SGML‐derived) documents.
character entity
A sequence representing a character with a special meaning in HTML (ex. &lt; for the less‐than symbol ‘<’), or that is not part of a document’s character set (ex. &copy; for the copyright symbol ‘©’). Character entities begin with ‘&’ and end with ‘;’.
content type
Content types denote what kind of file is being transmitted over the Web. Files are tagged according to the MIME standard, using type designations like text/plain, text/html, image/jpeg, or video/quicktime. Type designations that are not officially registered have an x- prefix, like audio/x-midi or application/x-javascript. The Internet Assigned Numbers Authority (IANA) maintains the official registry of content types.
Document Object Model. A standard object‐oriented way of representing HTML (and other SGML‐derived) documents as a tree of data objects with methods (procedures and functions) for manipulating their content.
Part of the structure of a HTML document. Elements usually begin with a start tag (ex. <em>) and end with an end tag (ex. </em>). However, HTML allows some tags to be omitted (ex. </p>).
See character entity.
end tag
A tag that represents the end of an element. In HTML, end tags begin with ‘</’ and end with ‘>’ (ex. </em>).
Hypertext Markup Language. The predominant document format that webpages are written in, including this one. It consists of text marked up with tags that define the document’s structure, load images, define links to other webpages, and so forth.
Hypertext Transfer Protocol. The predominant method for communicating on the World Wide Web. Basically, the manner in which a Web browser sends a message to a Web server requesting a file, and the Web server delivers either the file or an error message.
image map
Image maps link regions of an image to other webpages. When you click on different designated regions of the image, your Web browser loads different related webpages.
kibibyte (KiB)
A kibibyte is a unit of storage equal to exactly 1,024 bytes. Because kilobyte is used to mean either 1,000 bytes or 1,024 bytes, in 1999 the International Electrotechnical Commission defined a kibi‐ prefix unambiguously signifying 1,024. Rarely used except by pedantic nerds, like me.
Ligatures are two or more characters in sequence joined into a single unit. For example, the ae‐ligature (æ) and oe‐ligature (œ). They were used with movable type for thin characters that become illegible when run together, and in elegant handwriting. Ligatures are usually unnecessary in text designed for a computer screen, though they may be desirable when discussing historical documents or to accurately represent calligraphy.
mebibyte (MiB)
A mebibyte is a unit of storage equal to exactly 1,024 kibibytes, or 1,048,576 bytes. Because megabyte is used to mean 1,048,576 bytes, or 1,000,000 bytes, or even 1,024,000 bytes, in 1999 the International Electrotechnical Commission defined a mebi‐ prefix unambiguously signifying 1,048,576. Rarely used except by pedantic nerds, like me.
media type
See content type.
MIME content types are the standard way of denoting the nature of a file transmitted over the Web. Originally, the e‐mail protocol supported only plain text. MIME (Multipurpose Internet Mail Extensions) were originally devised to tag e‐mail with content types, allowing people to send images, audio, video, programs or other sorts of files. Over time, these content types were used to label content types in many other Internet protocols.
Standard Generalized Markup Language. A metalanguage for documents using tags to mark up elements. HTML is just one of many applications of SGML.
start tag
A tag that represents the beginning of an element (ex. <em>). Start tags contain the name of the element and may also include attributes.
style sheet
A style sheet defines styles (ex. fonts, margins, color, spacing) for a Web browser to use when rendering a webpage.
A token that represents the beginning or end of an element. A tag that begins an element is called a start tag, and one that ends and element is called an end tag. HTML tags begin with ‘<’ and end with ‘>’.
unknown‐character glyph
&#xFFFE; (), a distinct symbol designated to replace characters a Web browser cannot render. Web browsers that cannot render this symbol sometimes use other characters like ‘?’ to serve the purpose.
user agent
A program that accesses documents on the Web for any purpose. Web browsers, download managers, link checking programs, and indexing robots for search engines are all user agents.
Web browser
A program for accessing and rendering documents on the Web.
white space character
Characters in a document that are considered to separate words. In HTML 4.0, ASCII space characters, line breaks, tabs, form feeds, and zero‐width spaces are white space characters. One or more of these characters in the source document mark the end of one word and the beginning of another.
Extensible Hypertext Markup Language. A stricter remaking of HTML as an application of XML. The successor to HTML.
Extensible Markup Language. A meta­language for documents using tags to mark up elements. A simplified subset of SGML, without some of its complex features like tag omission. XHTML is just one application of XML.
zero‐width space
A space with no width at all, making it invisible. Some languages are not written with visible spaces between words, like Thai. Zero‐width spaces tell the Web browser where one word ends and another begins, or where a long word may be broken.