idattributter

An id attribute is a global attribute in HTML and other markup languages used to uniquely identify an element within a document. The value of an id can serve as a hook for styling, scripting, linking, and accessibility, and is commonly used by CSS selectors, JavaScript DOM methods, fragment identifiers in URLs, and ARIA attributes.

In HTML5 the id value must be unique within the document and must contain at least one character; it must not contain ASCII whitespace. Different environments treat case sensitivity differently: in XML-based documents (including XHTML served as XML) id values are case-sensitive, while HTML parsing typically treats them in a case-insensitive manner for some operations. Consumers such as CSS and JavaScript follow the rules of the underlying language and APIs, so consistent use is recommended.

Common uses include selecting an element with CSS via #id, accessing an element in script via document.getElementById, linking to a location with a URL fragment (example.com/page#section), and associating labels and controls for accessibility. Best practices encourage short, meaningful, and unique id names; avoid spaces and special characters that require escaping in CSS; prefer semantic names and reserve ids for unique elements rather than repeated styling—use classes for repeated styling. Care should be taken with dynamically generated content to prevent duplicate ids, which can break scripts and accessibility features.