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xmlns:xlink XLink Namespace Declaration

This is not an attribute, but the namespace pseudo-attribute. The value provides a URI to use for the XLink linking attributes (which has been set to a prefix of “xlink”). All namespace prefixes must be associated with a URL, and the prefix “xlink” has been set to the URL for the World Wide Web Consortium (W3C) XLink Recommendation.
FIXED (not settable) on many elements; click for list and usage

<abbrev>, <adoption>, <app>, <award-id>, <bio>, <chem-struct>, <collab>, <contrib>, <custom-meta>, <element-citation>, <email>, <ext-link>, <extended-by>, <funding-source>, <graphic>, <inline-graphic>, <inline-media>, <inline-supplementary-material>, <institution>, <issue-id>, <license>, <long-desc>, <media>, <mixed-citation>, <named-content>, <nlm-citation>, <pronunciation>, <pub-id>, <related-article>, <related-object>, <restricted-by>, <self-uri>, <standard>, <std>, <supplementary-material>, <uri>, <volume-id>

Value Meaning
http://www.w3.org/1999/xlink The prefix “xlink” has been set to the URL for the W3C XLink Recommendation.
Restriction This attribute has a fixed value which may not be changed in the XML document.
Tagged Sample

Inline graphic as a live link

...
<p><italic>Slow-front transient overvoltage (SFO)</italic>: 
 Transient overvoltage, usually unidirectional, with time to 
 peak 20 &#x3bc;s &lt; 
 <inline-formula>
  <alternatives>
   <tex-math notation="LaTeX">${T_p}$</tex-math>
   <inline-graphic xlink:href="1894_2015_mml_m1.eps"
     xmlns:xlink="http://www.w3.org/1999/xlink"/>
  </alternatives>
 </inline-formula> &#x2264; 5 000 &#x3bc;s, and 
 time&#xa0;to&#xa0;half&#xa0;value&#xa0;of&#xa0;a&#xa0;
 decreasing&#xa0;voltage 
 <inline-formula>
  <alternatives>
   <tex-math notation="LaTeX">${T_2}$</tex-math>
   <inline-graphic xlink:href="1894_2015_mml_m2.eps" 
     xmlns:xlink="http://www.w3.org/1999/xlink"/>
  </alternatives>
 </inline-formula> &#x2264; 20 ms.</p>
...
Related Resource
For a discussion on namespaces used in STS, see Namespacing NISO STS Elements.