In fact, checking the current code, in CheckTABLE(),
the code to check if a 'summary' is present has been
removed! It was REMOVED back on Nov 21, 2011,
commit/4fdc30c by @sideshowbarker, with the comment
"summary attribute is not required on table in HTML5".
The summary attribute on the table element is obsolete. Consider
describing the structure of the table in a caption element or in a
figure element containing the table element; or, simplify the structue
of the table so that no description is needed.
Differences Between HTML 4.01 and HTML5 - (on NOT border, cellpadding,
cellspacing, frame, rules, width) - Only the "summary" attribute is
supported in HTML5.
Tips and Notes - You should try not to use the "summary" attribute. If
the information in the "summary" attribute is important, you should use
a table caption or add a text paragraph before and/or after the table.
The main point is that any explanations about the purpose and
structure of a table that are needed for proper understanding should be
given in normal content before the table, or in the <caption> element.