An ETag is a signature attached a server response. If the client requests a URL (HTML page, image, style sheet, etc.) whose ETag is identical to the one it already has, the web server will reply that it does not need to download the resource and that it should use the one it already possesses. Using ETags saves huge amounts of bandwidth.

A repaint is a change in appearance of a DOM (Document Object Model) element, while a reflow is a change/recalculation of the position of the elements in the DOM. You should avoid triggering these two operations as they are resource-expensive, especially in terms of CPU usage.