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Hyperlink

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A hyperlink, often called a link, is a digital reference that provides direct access to data. Users follow these links by clicking a mouse or tapping a screen. A hyperlink can point to an entire document or a specific element within a document. When text contains these links, the document is referred to as hypertext. The specific text used to create the link is known as the anchor text.

Hyperlink example
Hyperlink example

Hyperlinks are essential for navigating digital information. They allow users to browse through vast amounts of data quickly and efficiently.

To understand how a hyperlink works, we must look at its structure. In the language of the web, HTML, a link is created using an anchor tag. This tag starts with "<a" and includes a hyperlink reference, or "href". This "href" attribute contains the URL, which is the digital address of the target. The code ends with an anchor closing tag, "</a".

hyperlinks scheme
hyperlinks scheme

When a browser renders this code, it usually displays the anchor text as underlined and colored. For instance, unvisited links often appear blue, while visited links may turn purple. This visual style is known as link decoration.

Hyperlinks come in several distinct types based on their function. Inline links display remote content without embedding the entire file. They might show a thumbnail or a low-resolution preview to save space. Anchor links, or anchor hyperlinks, are different because they point to a specific fragment within a document. A fragment is a small portion of text or a heading. In a URL, a hash character (#) precedes the name of the anchor to identify it.

Hyperlink example
Hyperlink example

There are also image hyperlinks, where a specific area of an image is clickable. This is sometimes called an image map.

Some links are more complex than a simple one-to-one connection. A fat link, also known as a one-to-many link, leads to multiple endpoints. This means a single link acts as a set-valued function. In some hypertext systems, links can even be bidirectional. This allows the link to be followed in two directions, so both ends act as both anchors and targets. More advanced systems use XLink to provide even greater functionality. XLink can describe links that work between different XML documents.

History shows that hyperlinks were vital to the birth of the modern internet. Tim Berners-Lee recognized the potential to link any information to any other information. This vision led to the creation of the World Wide Web. Because of this, hyperlinks are integral to how we use the web today.

SRI ARC Engelbart Nov 1969
SRI ARC Engelbart Nov 1969

While humans browse the web, programs can follow links too. A program that traverses hypertext to gather documents is called a web spider or a crawler.

Maintaining links over time presents a significant challenge. Some URLs are designed to be permanent, known as permalinks. These are intended to remain unchanged for many years to prevent "link rot." Link persistence is especially important in scientific literature to preserve public knowledge. A 2013 study analyzed 15,000 links in scientific abstracts. The study found that the median lifespan of a web page was only 9.3 years. Additionally, only 62% of those pages were archived.

Hyperlinks connect to many different technological systems and files. In Windows, an Internet shortcut is a text file with a .url extension. macOS uses a different file format called .webloc, which uses XML syntax. Linux systems with a desktop environment use .desktop files. There are even cross-platform ways to implement links using .html files. Whether they are used for email addresses, tables of contents, or bibliographies, hyperlinks turn isolated data into a connected web of knowledge.

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File:Hyperlink example.svg
Hyperlink example.svg
File:hyperlinks_scheme.svg
hyperlinks_scheme.svg
File:SRI ARC Engelbart Nov 1969.jpg
SRI ARC Engelbart Nov 1969.jpg
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