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Return the first registered implementation that has the desired features, or null if none is found. 
 
"Experimental - subject to change" 
 - Parameters:
 - 
  
     | features  | A string that specifies which features are required. This is a space separated list in which each feature is specified by its name optionally followed by a space and a version number. This is something like: "XML 1.0 Traversal 2.0"  |  
   
  
- Returns:
 - An implementation that has the desired features, or 
null if this source has none.   
- Since:
 - DOM Level 3 
  
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