Using display:table-cell without containing display:table

Yes, it is valid. Read 17.2.1 of CSS2 spec regarding anonymous table objects. More specifically, these sections…

Generating missing child wrappers:

1. If a child C of a ‘table’ or
   ‘inline-table’ box is not a proper
   table child, then generate an
   anonymous ‘table-row’ box around C and
   all consecutive siblings of C that are
   not proper table children.
2. If a child
   C of a row group box is not a
   ‘table-row’ box, then generate an
   anonymous ‘table-row’ box around C and
   all consecutive siblings of C that are
   not ‘table-row’ boxes.
3. If a child C of
   a ‘table-row’ box is not a
   ‘table-cell’, then generate an
   anonymous ‘table-cell’ box around C
   and all consecutive siblings of C that
   are not ‘table-cell’ boxes.

Generate missing parents:

1. For each ‘table-cell’ box C in a
   sequence of consecutive internal table
   and ‘table-caption’ siblings, if C’s
   parent is not a ‘table-row’ then
   generate an anonymous ‘table-row’ box
   around C and all consecutive siblings
   of C that are ‘table-cell’ boxes.

2. For
   each proper table child C in a
   sequence of consecutive proper table
   children, if C is misparented then
   generate an anonymous ‘table’ or
   ‘inline-table’ box T around C and all
   consecutive siblings of C that are
   proper table children. (If C’s parent
   is an ‘inline’ box, then T must be an
   ‘inline-table’ box; otherwise it must
   be a ‘table’ box.)

   • A ‘table-row’ is
     misparented if its parent is neither a
     row group box nor a ‘table’ or
     ‘inline-table’ box.
   • A ‘table-column’
     box is misparented if its parent is
     neither a ‘table-column-group’ box nor
     a ‘table’ or ‘inline-table’ box.
   • A row
     group box, ‘table-column-group’ box,
     or ‘table-caption’ box is misparented
     if its parent is neither a ‘table’ box
     nor an ‘inline-table’ box.