it would render correctly, yes, but wouldn't pass html validation, so I'm guessing the browser engine stutters on it a bit.
IIRC the 'error' is that the table tag should be closed before the headertemplate tag in order to be a well formed html doc.
The browser eventually sorts it out, but making it well formed there's less to think about for the engine.
If you move things around as in my above post, the table encloses the HeaderTemplate and asp:Repeater
fixing it seems to have made the page loads quicker