Hopefully someone will chime in with more helpful advice, but I suspect there will remain some thorny limitations on tabbing in WD compared with FMPro.
The "FileMaker 13 WebDirect Guide" cautions that "Many tasks such as CSS cascading, determining focus, propagating events, and tabbing are ultimately controlled by the web browser and behave differently than in FileMaker Pro. For example, FileMaker WebDirect intercepts Tab key presses and sends them to the Database Server to determine the next object in the layout’s tab order. But at some point, pressing the Tab key exits the webpage and enters the web browser’s interface (for example, the address bar)." and also "The tab order might vary from the tab order in FileMaker Pro. Different browsers might support the tab order in a different manner."
Possibly some carefully placed script triggers could help?
I know, not very helpful I am today.
We are seeing the same thing using Safari. Not a deal breaker, but will confuse entry workers. You can tab into the first field within a portal, but after an entry and another tab, WD must be creating the new related record and when it returns focus to your screen the field position is forgotten. We are able to tab again and end up in the third portal field, but the second is overlooked.