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xml:base Base

Provides a base URI for identifiers in the XML document or a part of the XML document. Within this document, or 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://jats.nlm.nih.gov/”, a URI inside the document of “publishing/rationale.html” would be processed as if it were “https://jats.nlm.nih.gov/publishing/rationale.html”. Values of @xml:base are inherited, 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>, <ack>, <addr-line>, <address>, <aff>, <aff-alternatives>, <ali:free_to_read>, <ali:license_ref>, <alt-text>, <alt-title>, <alternatives>, <annotation>, <anonymous>, <answer>, <answer-set>, <app>, <app-group>, <array>, <article>, <article-meta>, <article-title>, <article-version>, <attrib>, <author-comment>, <award-desc>, <award-group>, <award-id>, <award-name>, <back>, <bio>, <body>, <bold>, <boxed-text>, <break>, <caption>, <chapter-title>, <chem-struct>, <chem-struct-wrap>, <citation-alternatives>, <city>, <code>, <col>, <colgroup>, <collab>, <collab-alternatives>, <comment>, <compound-kwd>, <compound-kwd-part>, <compound-subject>, <compound-subject-part>, <conf-acronym>, <conf-date>, <conf-loc>, <conf-name>, <conf-num>, <conf-sponsor>, <conf-theme>, <conference>, <contrib>, <contrib-group>, <contrib-id>, <contributed-resource-group>, <copyright-holder>, <copyright-statement>, <copyright-year>, <country>, <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>, <edition>, <element-citation>, <elocation-id>, <email>, <era>, <etal>, <explanation>, <ext-link>, <extended-by>, <fax>, <fig>, <fixed-case>, <fn>, <fn-group>, <fpage>, <front>, <funding-group>, <funding-source>, <funding-statement>, <given-names>, <glossary>, <glyph-data>, <glyph-ref>, <gov>, <graphic>, <hr>, <index-term>, <index-term-range-end>, <inline-formula>, <inline-graphic>, <inline-media>, <inline-supplementary-material>, <institution>, <institution-id>, <institution-wrap>, <isbn>, <issn>, <issn-l>, <issue>, <issue-id>, <issue-part>, <issue-title>, <italic>, <journal-id>, <kwd>, <kwd-group>, <label>, <license>, <license-p>, <list>, <list-item>, <long-desc>, <lpage>, <media>, <meta-name>, <meta-value>, <mixed-citation>, <monospace>, <month>, <name>, <name-alternatives>, <named-content>, <nested-kwd>, <nlm-citation>, <object-id>, <on-behalf-of>, <open-access>, <option>, <overline>, <p>, <page-count>, <page-range>, <part-title>, <patent>, <permissions>, <person-group>, <phone>, <postal-code>, <prefix>, <preformat>, <price>, <principal-award-recipient>, <principal-investigator>, <private-char>, <processing-meta>, <product>, <pub-id>, <publisher-loc>, <publisher-name>, <question>, <question-preamble>, <question-wrap>, <question-wrap-group>, <rb>, <ref>, <ref-list>, <related-article>, <related-object>, <resource-group>, <resource-id>, <resource-name>, <resource-wrap>, <restricted-by>, <role>, <roman>, <rt>, <ruby>, <sans-serif>, <sc>, <season>, <sec>, <sec-meta>, <see>, <see-also>, <self-uri>, <series>, <size>, <source>, <speaker>, <speech>, <state>, <statement>, <std>, <std-organization>, <strike>, <string-date>, <string-name>, <styled-content>, <sub>, <subj-group>, <subject>, <subtitle>, <suffix>, <sup>, <supplement>, <supplementary-material>, <support-description>, <support-group>, <support-source>, <surname>, <table>, <table-wrap>, <table-wrap-foot>, <target>, <tbody>, <td>, <term>, <term-head>, <textual-form>, <tfoot>, <th>, <thead>, <time-stamp>, <title>, <title-group>, <tr>, <trans-source>, <trans-title>, <underline>, <uri>, <verse-group>, <verse-line>, <version>, <volume>, <volume-id>, <volume-series>, <xref>, <year>

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