Thank you for posting.
As many of FileMaker Pro features, it seems that this new one is not supported over IWP.
But true, i did'nt find any documentation about this lack indeed !
Thank you for the post.
The documented limitations of using a screen reader with FileMaker's Instant Web Publishing can be found here:
Page 4, Criteria (d) states that layout object tooltips are among the HTML Elements that "do not provide any information about the control."