The web TabEngine is, frankly, a mess. A lot of work needs to be done in that regard. There are a few open Feedback Cases that I hope will be fixed soon-ish.
<https://xojo.com/issue/67914>
<https://xojo.com/issue/66766>
<https://xojo.com/issue/65762>
<https://xojo.com/issue/61609>
I’ve taken to writing the JavaScript to set the taborder to controls then forcibly breaking the TabEngine. Maybe @Ricardo_Cruz can tackle this for an upcoming release?