Your guestbook's handling of HTML tags is determined in a set of lines in guestbook.pl, and look like this:
# allow html tags - 0=no; 1=yes; 2=translate $html_tags = "2";
Your choices are no ($html_tags = "0";), yes ($html_tags = "1";), and translate ($html_tags = "2";).
No ignores HTML tags. For example, Your site is <strong>great</strong>, would appear as your site is great.
Yes allows and interprets HTML tags. Your site is <strong>great</strong> would appear as Your site is great.
Translate inserts the entry into your guestbook exactly as typed. Your site is <strong>great</strong> will appear as Your site is <strong>great</strong>.