LONGTEXT is probably excessive -- it allows data sizes up to 4GB.
MEDIUMTEXT allows 16MB.
TEXT allows 64KB.
TINYTEXT allows only 255 characters.
Very few HTML documents are going to be 64KB in size, so a TEXT ought to be suitable for you. If you really want to be safe, go for a MEDIUMTEXT.
Reference: http://dev.mysql.com/doc/refman/5.0/en/string-type-overview.html