Although the changelog may seem relatively short, there's one line in specific we've put a lot of effort into: Major improvements to the CSS parser.
This is actually a complete rewrite of the CSS functionality, resulting in much better interpretation of CSS syntax.
You'll find these changes in iText's XML Worker. The HTMLWorker class that was shipped with iText core was finally deprecated.
Read more: http://itextpdf.com/release/iText552