Note: There is an extended journal version.
BibTeX:
@inproceedings{Nipkow-FSTTCS-96,author={Tobias Nipkow}, title={Winskel is (almost) Right: Towards a Mechanized Semantics Textbook}, booktitle= {Foundations of Software Technology and Theoretical Computer Science}, editor={V. Chandru and V. Vinay}, publisher=Springer,series=LNCS,volume=1180,year=1996,pages={180--192}, url={http://www4.informatik.tu-muenchen.de/~nipkow/pubs/fsttcs96.dvi.gz}}