Commonly used namespaces in HTML include:

http://www.w3.org/XML/1998/namespace

the URI for the XML base namespace; associated with the implicitly declared reserved prefix xml:

http://www.w3.org/2000/xmlns/

the namespace URI for XML namespace declarations; associated with the implicitly declared reserved prefix xmlns:

http://www.w3.org/1999/xhtml

the URI for the HTML namespace, the same namespace URI defined by the 2000-2010 Recommendations from the W3C HTML Working Group; implicitly declared as the default namespace for unprefixed tag names when document is being parsed as HTML, but explicitly coding it is recommended for cases when document is being parsed as xHTML or XML

http://www.w3.org/1998/Math/MathML

the MathML namespace for MathML tags

http://www.w3.org/1999/02/22-rdf-syntax-ns#

the RDF namespace for RDF tags

http://www.w3.org/2000/01/rdf-schema#

the RDF schema namespace

http://www.w3.org/2000/svg

the SVG namespace for SVG tags

http://www.w3.org/1999/xlink

the XLink namespace

http://www.w3.org/2001/XMLSchema

the namespace for XML Schema Definitions, which are XML documents that replace non-XML DTDs

http://www.w3.org/2001/XMLSchema-instance

the namespace for XML Schema instance documents, which can be used to specify whether the data for a field is binary (possibly encrypted) or plain text:

<span id="masked-credit-card-number" xsi:type="xsd:string">4321 **** **** 8765</span>
<span id="encrypted-credit-card-number" xsi:type="xsd:base64Binary">BAM0NComFzC2TOsmRzW0NTueQU==</span>

http://www.w3.org/1999/XSL/Transform

the namespace for XSL style sheets, which is not used in HTML documents themselves, but is used in the style sheet documents that are referenced by the xml-stylesheet instruction and contain XSLT elements intermingled with HTML code.