In TEX each cell of an halign table resides within its own
implied brace group. Because TTH does not implement this implied
group, errors can arise. Even HTML table errors that lead to parse
errors with XML parsers can arise when the cells have boxes in them.
If this happens, the fix is to put an explicit brace group round the
offending cell in the template line like this example:
\halign{#\quad\hfil &{\vbox{\hsize\0.5\hsize #}}\cr
1 & V-box material\cr
}