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xlink:href Href (XLink Linking Mechanism)

An address or identifier that permits an XLink application to find a remote resource, for example, a URI (such as a URL or filename) that links to an online site associated with the author or the online definition of a term. As specified in XML Linking Language (XLink) Version 1.0, the value of an @xlink:href must be a valid URI. (See https://www.w3.org/TR/xlink/.)

Usage/Remarks

Of the many linking attributes that may be used, @xlink:href is the most important, since it provides the link. The attribute is required for those elements which must link, and it is optional on other elements only in the sense that an element need not link. If an element links, this attribute is required.
REQUIRED on elements: <graphic>, <inline-graphic>, <media>
Value Meaning Behavior
Text, numbers, or special characters A pointer (which may be a URL) to a resource. Can be used to make this element a live link to a related site.
Restriction This attribute is required; it must be provided if the element is used.
OPTIONAL on many elements; click for list and usage

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

Value Meaning Behavior
Text, numbers, or special characters A pointer (which may be a URL) to a resource. Can be used to make this element a live link to a related site.
Restriction This is an optional attribute; there is no default.
Tagged Samples
To link to book included as bibliographic reference 
...
<ref id="bibr40">
 <label>40</label>
 <mixed-citation><collab>World Health Organization</collab>, 
  <collab>World Bank</collab>. (<year iso-8601-date="2011">2011</year>), 
  <source>World Report on Disability</source>, 
  <publisher-loc>Geneva</publisher-loc>, available at: 
  <ext-link 
    xlink:href="http://www.who.int/disabilities/world_report/2011/en/index.html"   
  >http://www.who.int/disabilities/world_report/2011/en/index.html</ext-link
  ></mixed-citation>
 </ref>
...
To link to graphic image 
...
<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>
...