Reported by Simon Sapin
In fixed table layout, column widths are determined by the cells of the first row. But is that first row in source order or the first row in display order? Observe that 17.2 (The CSS table model) says:
# table-header-group (In HTML: THEAD)
# Like 'table-row-group', but for visual formatting, the row
# group is always displayed before all other rows and row groups
# and after any top captions.