Grammar taken from the GraphViz Web site:

cell: <TD> (text* | table | <IMG/>) </TD>
text : string
     | <BR/>
     | <FONT> text* </FONT>
     | <I> text* </I>
     | <B> text* </B>
     | <U> text* </U>
     | <O> text* </O>
     | <SUB> text* </SUB>
     | <SUP> text* </SUP>
     | <S> text* </S>
row: <TR> cell ((<VR/>)? cells)? </TR>
table : <FONT> table </FONT>
      | <TABLE> (row (<HR/>)?)* </TABLE>

See also

- http://www.graphviz.org/content/node-shapes#html

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