From https://lists.w3.org/Archives/Public/www-style/2012Aug/0298.html:

• RESOLVED: Errata CSS2.1 to say that 'overflow' on a table element applies to the table box (not table wrapper box); and that values other than 'hidden' are treated as 'visible'.

CSS 2.1 errata: https://www.w3.org/Style/css2-updates/REC-CSS2-20110607-errata.html#s.11.1.1b

Note that this errata only appears to apply the second half of that resolution.