CSS is thought out for it to be possible to implement a fast layout engine. This requires a few concessions. One concession is that it (mostly) requires that la

CSS is thought out for it to be possible to implement a fast layout engine. This requires a few concessions. One concession is that it (mostly) requires that layout defined by a partial document is the same when the document is complete (to allow for documents to be laid out on the screen as they come in from the slow network). A placement: middle, without a rigidly defined height, would require the layout engine to know about document information after the element requesting a middle placement.