Thanks to everyone, I have solved the problem. I succeeded by installing the package "latex2html", which includes the file "html.sty". NOTE THAT THE FILE "html.sty" LISTED IN A PREVIOUS MAIL IS NOT VALID BECAUSE I GOT THE FILE "typehtml.sty" FROM ANOTHER SYSTEM AND RENAMED IT TO "html.sty" TO CHECK IF THE COMPILATION PROBLEM DESAPPEARED. BUT IT DIDN'T WORK. APOLOGIES FOR THE ERROR. Thanks, Manuel Arenaz