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.