The following messages are reported by "tidy" on the generated GridBagLayout.html file in the API docs.

line 281 column 10 - Warning: The summary attribute on the <table> element is obsolete in HTML5
line 355 column 10 - Warning: The summary attribute on the <table> element is obsolete in HTML5
line 409 column 10 - Warning: The summary attribute on the <table> element is obsolete in HTML5
line 281 column 2 - Warning: <center> element removed from HTML5
line 355 column 2 - Warning: <center> element removed from HTML5
line 409 column 2 - Warning: <center> element removed from HTML5
Info: Document content looks like HTML5
Tidy found 6 warnings and 0 errors!

These issues should be addressed so that the generated files contain valid HTML 5.