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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 nullif this source has none.
 Since:DOM Level 3 
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