SET MARKUP HTML ON SPOOL ON generates an HTML page for each subsequently spooled file. The HTML tags in a spool file are closed when SPOOL OFF is executed or SQL*Plus exits.
You can use SET MARKUP HTML ON SPOOL OFF to generate HTML output suitable for embedding in an existing web page. HTML output generated this way has no or tags.
SET MARKUP HTML ON SPOOL ON only specifies that SQL*Plus output will be HTML encoded, it does not create or begin writing to an output file. You must use the SQL*Plus SPOOL command to start generation of a spool file. This file then has HTML tags including and .
You use SPOOL OFF or EXIT to append final HTML tags to the spool file and then close it. If you enter another SPOOL filename command, the current spool file is closed as for SPOOL OFF or EXIT, and a new HTML spool file with the specified name is created.
So generating output will be as follows: (Run following on SQLPLUS):
set markup html on spool on
select * from v$session;