InvalidOperationException: 'ColumnSpan must be at least 1.' is thrown when importing HTML document with table column containing colspan="0".

The colspan="0" has special meaning according to the HTML specification http://www.w3.org/TR/html401/struct/tables.html:
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colspan = number [CN]

This attribute specifies the number of columns spanned by the current cell. The default value of this attribute is one ("1"). The value zero ("0") means that the cell spans all columns from the current column to the last column of the column group (COLGROUP) in which the cell is defined.
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Workaround: Preprocess the HTML, and delete all occurrences of colspan="0".