the author describes how their render-once approach lets them implement CSS in a simpler way since they don't need to retain information for arbitrary dynamic changes in the stylesheets and content
I guess this implies that rescaling the window, or rotating your phone, will not update the view. Then you'd have to reload the page. That trade-off seems okay to me.
Yeah, the layout would still resize but it could be non-ideal in some cases as it would be based on media queries for another width.
For the rotation I could process a second layout on the background and switch to it instantly if rotated. Similarly hover effects will be limited. Things affecting visibility of the blocks/layers should work (for menus), small adjustments of layouted text too, but anything that is more complicated won't be. It currently uses a hardcoded hover effect for links.
they also probably don't implement most of CSS