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xml:base Base

Provides a base URI for identifiers in the XML-tagged standards document or a part of an XML-tagged standards document. Within this standards document, or a portion of the document, all URIs are assumed to be built on the “base” provided as the value of this attribute. For example, if a document has an @xml:base of “https://mulberrytech.com/”, a URI inside the document of “scope/rationale.html” would be processed as if it were “https://mulberrytech.com/scope/rationale.html”. Values of @xml:base are inherited from the closest base, meaning that if there is an @xml:base on a document and on a part of the document, uses inside the part of the document take the value of the part, not the whole.
OPTIONAL on many elements; click for list and usage

<abbrev>, <abstract>, <access-date>, <accrediting-organization>, <ack>, <addr-line>, <address>, <adoption>, <aff>, <aff-alternatives>, <ali:free_to_read>, <ali:license_ref>, <alt-text>, <alt-title>, <alternatives>, <annotation>, <anonymous>, <app>, <app-group>, <array>, <article-title>, <attrib>, <author-comment>, <authorization>, <award-id>, <back>, <bio>, <body>, <bold>, <boxed-text>, <break>, <caption>, <chapter-title>, <chem-struct>, <chem-struct-wrap>, <citation-alternatives>, <city>, <code>, <col>, <colgroup>, <collab>, <collab-alternatives>, <comm-ref>, <comm-ref-group>, <comment>, <compl>, <compl-title-wrap>, <compound-kwd>, <compound-kwd-part>, <compound-subject>, <compound-subject-part>, <conf-acronym>, <conf-date>, <conf-loc>, <conf-name>, <conf-sponsor>, <content-language>, <contrib>, <contrib-group>, <contrib-id>, <copyright-holder>, <copyright-statement>, <copyright-year>, <count>, <country>, <counts>, <custom-meta>, <custom-meta-group>, <data-title>, <date>, <date-in-citation>, <day>, <def>, <def-head>, <def-item>, <def-list>, <degrees>, <disp-formula>, <disp-formula-group>, <disp-quote>, <doc-ident>, <doc-number>, <doc-ref>, <doc-type>, <editing-instruction>, <edition>, <element-citation>, <elocation-id>, <email>, <equation-count>, <era>, <etal>, <ext-link>, <fax>, <fig>, <fig-count>, <fig-group>, <fixed-case>, <fn>, <fn-group>, <fpage>, <full>, <funding-source>, <given-names>, <glossary>, <glyph-data>, <glyph-ref>, <gov>, <graphic>, <hr>, <ics>, <ics-desc>, <ics-wrap>, <index>, <index-div>, <index-entry>, <index-group>, <index-term>, <index-term-range-end>, <index-title-group>, <inline-formula>, <inline-graphic>, <inline-supplementary-material>, <institution>, <institution-id>, <institution-wrap>, <intro>, <intro-title-wrap>, <is-proof>, <isbn>, <iso-meta>, <issn>, <issn-l>, <issue>, <issue-id>, <issue-part>, <issue-title>, <italic>, <journal-id>, <kwd>, <kwd-group>, <label>, <language>, <license>, <license-p>, <list>, <list-item>, <long-desc>, <lpage>, <main>, <main-title-wrap>, <media>, <meta-date>, <meta-name>, <meta-note>, <meta-value>, <milestone-end>, <milestone-start>, <mixed-citation>, <monospace>, <month>, <name>, <name-alternatives>, <named-content>, <nat-meta>, <nav-pointer>, <nav-pointer-group>, <nested-kwd>, <nlm-citation>, <non-normative-example>, <non-normative-note>, <normative-example>, <normative-note>, <notes>, <notes-group>, <num>, <oasis:table>, <oasis:tbody>, <object-id>, <on-behalf-of>, <open-access>, <originator>, <overline>, <p>, <page-count>, <page-range>, <part-number>, <part-of-speech>, <part-title>, <patent>, <permissions>, <person-group>, <phone>, <postal-code>, <prefix>, <preformat>, <price>, <private-char>, <proj-id>, <pronunciation>, <pub-date>, <pub-id>, <publisher-loc>, <publisher-name>, <rb>, <ref>, <ref-count>, <ref-list>, <reg-meta>, <related-article>, <related-object>, <related-term>, <release-date>, <release-version>, <release-version-id>, <role>, <roman>, <rt>, <ruby>, <sans-serif>, <sc>, <sdo>, <season>, <sec>, <secretariat>, <see>, <see-also>, <see-also-entry>, <see-entry>, <self-uri>, <series>, <sig>, <sig-block>, <size>, <source>, <speaker>, <speech>, <standard>, <state>, <statement>, <std>, <std-doc-meta>, <std-id>, <std-id-group>, <std-ident>, <std-meta>, <std-org>, <std-org-abbrev>, <std-org-group>, <std-org-name>, <std-ref>, <std-xref>, <strike>, <string-name>, <styled-content>, <sub>, <sub-part>, <subj-group>, <subject>, <subtitle>, <suffix>, <sup>, <suppl-number>, <suppl-type>, <suppl-version>, <supplement>, <supplementary-material>, <surname>, <table>, <table-count>, <table-wrap>, <table-wrap-foot>, <table-wrap-group>, <target>, <tbody>, <tbx:crossReference>, <tbx:definition>, <tbx:entailedTerm>, <tbx:example>, <tbx:externalCrossReference>, <tbx:geographicalUsage>, <tbx:grammaticalGender>, <tbx:grammaticalNumber>, <tbx:langSet>, <tbx:normativeAuthorization>, <tbx:note>, <tbx:partOfSpeech>, <tbx:pronunciation>, <tbx:see>, <tbx:source>, <tbx:subjectField>, <tbx:term>, <tbx:termEntry>, <tbx:termType>, <tbx:tig>, <tbx:usageNote>, <tbx:xGraphic>, <tbx:xMathML>, <tbx:xSource>, <td>, <term>, <term-display>, <term-head>, <term-sec>, <term-source>, <tex-math>, <textual-form>, <tfoot>, <th>, <thead>, <time-stamp>, <title>, <title-wrap>, <toc>, <toc-div>, <toc-entry>, <toc-group>, <toc-title-group>, <tr>, <trans-source>, <trans-subtitle>, <trans-title>, <trans-title-group>, <underline>, <uri>, <urn>, <verse-group>, <verse-line>, <version>, <volume>, <volume-id>, <volume-series>, <wi-number>, <word-count>, <xref>, <year>

Value Meaning
Text, numbers, or special characters The base URI.
Restriction This is an optional attribute; there is no default.