It makes sense to sign parts of web pages (as for the mentioned PICS tags-- aren't those the voluntary content rating tags?). But for most serious sites the content is created by CGI applications which would have to sign them on the fly-- there's no protection in that. For linked static files, signing each of them can't hurt, except that you'd need a client that could de-sign them in the download process. Better would be a signed text file containing the approved MD5 or SHA1 hashes for the downloadable files or libraries. For files where integrity is important this also verifies that no errors were introduced.