[pope52]

They include an acknowledgment to Arc90 and Readability on their acknowledgements page.

file:///Applications/Safari.app/Contents/Resources/Acknowledgments.html

[skeletonjelly]

Can you post it or pastebin for those who don't have it? Thanks!

[snprbob86]

  Acknowledgments
  Portions of this Apple Software may utilize the following copyrighted material, the use of which is hereby acknowledged.

Then after several other acknowledgements:

  Arc90 ( Readability )
  Copyright © Arc90 Inc.  Readability is licensed under the Apache License, Version 2.0.  

Followed by a full copy of the Apache License, V2

[hboon]

It reads:

  Arc90 ( Readability )
  Copyright © Arc90 Inc.  Readability is licensed under the Apache License, Version 2.0.

Followed by the Apache 2 license.