Fieldset inside fieldset

<!ELEMENT FIELDSET - - (#PCDATA,LEGEND,(%flow;)*) -- form control group -->

Fieldset must contain a legend followed by zero or more %flow elements

<!ENTITY % flow "%block; | %inline;">

Flow includes %block

<!ENTITY % block
 "P | %heading; | %list; | %preformatted; | DL | DIV | NOSCRIPT |
  BLOCKQUOTE | FORM | HR | TABLE | FIELDSET | ADDRESS">

Block includes FIELDSET

It is fine.