XML namespace directory

The namespace every sitemap.xml and sitemap index must declare, almost always as the default. Declare the https:// form and crawlers reject the file.

The namespace for Atom feeds, and for the <atom:link rel="self"> element that well-formed RSS feeds borrow to point at their own URL.

The vocabulary a schema document is written in — the xs: elements plus the built-in datatypes an XSD refers to. Bound to xs: or xsd: by convention.

Four attributes an instance document uses to point at its schema or declare a nil value: xsi:schemaLocation, xsi:type and xsi:nil among them.

The SOAP 1.1 envelope namespace, and distinct from SOAP 1.2: the version is encoded in the URI, so compare URIs and never the soap: prefix.

The SOAP 1.2 envelope namespace, and the reason a 1.1 endpoint answers a 1.2 message with a VersionMismatch fault rather than a parse error.

The service-description namespace for SOAP web services, and still the version almost every deployed WSDL uses — 2.0 never displaced it.

The transformation-language namespace, unchanged across XSLT 1.0, 2.0 and 3.0 — the version lives in an attribute, not the URI, so 1999 is correct.

Scalable Vector Graphics. A standalone .svg file must declare this on its root element or browsers render nothing; inline SVG in HTML need not.

XHTML markup — and, in sitemaps, the namespace that carries <xhtml:link> hreflang alternates, which is why stripping it destroys hreflang coverage.

Fifteen general-purpose metadata terms — creator, date, title, subject, rights and friends — reused inside RSS, RDF and publishing formats alike.

Supplies <content:encoded>, the element feeds use to carry a full HTML article body when RSS's own <description> holds only a summary.

The de facto podcast vocabulary: episode and season numbers, artwork, categories, explicit flags and durations. Every major directory reads it.

Declares the images belonging to each <url> entry in a sitemap, so a crawler can associate them with the page that hosts them without parsing it.

Publication name, language and publication date for news articles in a sitemap — the one sitemap type where a stale file is worse than no file.

Thumbnail, duration, player and content locations for video in a sitemap. Several child elements are required together, or the entry is dropped.

The RDF graph syntax in XML, and the namespace that identifies an RSS 1.0 document — whose root element is <rdf:RDF> rather than <rss>.

Describes a search endpoint so a client can build query URLs, and adds totalResults, startIndex and itemsPerPage to search-result feeds.

Bound to the xml: prefix implicitly and permanently, and the source of xml:lang, xml:space, xml:base and xml:id. Never declare it, never rebind it.

The localisation interchange format that moves translatable content between a content system and a translation vendor. Still more used than 2.x.

The root namespace for a UBL invoice, the basis of most European e-invoicing mandates. A UBL document declares this one plus cbc: and cac:.

Every leaf value in a UBL document — identifiers, dates, amounts, quantities, codes. If it holds a value rather than a structure, it is cbc:.

Every structure in a UBL document — parties, addresses, lines, tax totals. If it groups other elements rather than holding a value, it is cac:.

The de facto way to attach images, video and audio to a feed item with dimensions, type and thumbnails — far richer than RSS's own <enclosure>.

Digital signatures embedded in the XML they sign. The namespace behind SAML assertions, signed SOAP messages and e-invoicing mandates.

Authentication and signing headers for SOAP — the UsernameToken and BinarySecurityToken that enterprise services expect in the envelope header.

The generic way to express a hyperlink in any XML vocabulary. Most visible today as xlink:href in SVG, WSDL and DocBook.

The default namespace on pom.xml, covering Maven project coordinates, dependencies, build plugins, profiles, and repositories.

The namespace for SAML requests, responses, status codes, logout messages, and other protocol-level exchanges.

The identity-bearing half of SAML: assertions, issuers, subjects, conditions, authentication statements, and attributes.

The semantic vocabulary for DocBook 5 books, articles, sections, procedures, references, and technical inline markup.

The namespace for mathematical notation in standalone MathML and embedded XML vocabularies such as JATS and EPUB.

The paged-media vocabulary used by FO processors for page masters, flows, blocks, tables, inline content, and pagination properties.