If we're speaking of graphically drawing things out, the browser is already doing this. The server just sends the raw data about the objects (coordinates, basically), then the browser renders everything through SVG data. The selectable road object is actually separate from the road that you see. It is an invisible vector object that gets manipulated as you select it or hover over it.
Because all objects are essentially data, and not graphics, we can create nifty add-ons, like timbones' color hightlights script, that can manipulate segments.
The only graphics that the server servers up is the Bing imagery, which comes from Bing servers, so it doesn't present an additional load on the Waze servers.
We are saying the same thing I think. What we see visually for the road layer, is a png, aligned with the actual SVG layer. What I'd like to see is the SVG layer only to be loaded, and have the browser draw the actual visible layer too.
In terms of the browser save causing a tile refresh, that is a capability of the current infrastructure and was one of the major features of the upgrade the US got a few months ago, and the World got this past week.
However, we do not want it to happen this quickly, at least, not so the app can download it right away or by default. Can you imagine a tile being saved 20 times in 5 minutes and the app downloading the tile 20 times? That would crush both the infrastrucutre network bandwidth and kill our data plan limits.
It may be able to do that, but before we do, we need better, more restrictive default app tile download behavior.