getElementsByTagNameNSnamespaceURI

getElementsByTagName is a DOM method available on the Document and Element interfaces. It retrieves all descendant elements with a given tag name and returns them as a collection. In HTML documents the tag name matching is case-insensitive, and the result is a live collection that updates as the document changes.

Usage: document.getElementsByTagName('div') returns all div elements in the document; document.getElementsByTagName('*') returns all descendant elements; you can call this on a specific element to search within that element's subtree. The return value is a live HTMLCollection, meaning that if elements are added or removed that match, the collection reflects the change automatically. Elements can be accessed by index or iterated with a loop; the length property indicates the number of matched elements.

Variants and considerations: getElementsByTagNameNS(namespaceURI, localName) provides namespace-aware tagging, commonly used in XML documents. In most HTML contexts this is not required. The method is widely supported across browsers, but can be slower on large documents, and its live nature requires consideration when mutating the DOM during iteration. As an alternative for static, CSS-based querying, querySelectorAll returns a static NodeList and supports complex selectors.