We present the CIFFWEB system, an innovative tool for the verification of web sites, relying upon abductive logic programming. The system allows the user to define rules that a web site should fulfill by using (a fragment of) the query language Xcerpt. The rules are translated into abductive logic programs with constraints and their fulfillment is checked through the CIFF abductive proof procedure. 1 Web Checking Rules The exponential growth of the WWW raises the question of maintaining and repairing automatically web pages at both structural and data level. Our web checking rules are characterized by using (a fragment of) the query language Xcerpt [2] for expressing complex queries in a natural syntax. The following is an example of a XML page [P1] about a theater company (left) and a rule expressing that no director should occur twice in the director list [R1] (right). %%% directorindex.xml GOAL all error [var D,"double director"] <dirlist> FROM <dir>dir1</di...