Nothing. The thing is that design time VS.NET checks for obvious html errors. It
checks for correctness of html tags and which one can (by specification) include
which elements... So a table  definitely doesn't permit anything else than tr,
tbody and maybe something else. repeater  or header template or other exotic
elements are definitely not permitted. The thing is that when you save your page
it will work correctly and you won't get any runtime error. And in fact the page
will also compile correctly without any errors.

Actually I hate VS.NET for these problems. It deletes code, it changes styles
and things like that. I hate that, so I disabled all this to make it at least
usable. I rather write my complex tables than draw them inside designer...

Maybe just a hint. Inside your header put  only Table element  no TRs or TDs.
Actually you're making by specification a not error-free table. Each row should
incorporate same number of cells.

You could also ommit header and footer templates if you put TABLE outside
repeater... Maybe the design error  won't happen. and your repeater won't have
any header and footer which will execute faster.