If you've got planets in space, in a realistic way, this probably means that:
- Planets are extremely large
- There is a lot of space in between them
This means that orthographic projections (like you might use for a 2d tile game) are very unhelpful. So you will have to use some kind of perspective view.
Which is fine, you just need to determine what transformations are necessary to have the camera facing the right direction and showing objects the appropriate size for their distance. Obviously, a realistic view of e.g. planet Mars from earth would typically be very small (e.g. a small dot), as it appears in the sky. But depending on the simulation, you might be standing on, say, Ganymeade looking at Jupiter, where it would take up half the sky (or something). There are plenty of applications which already do this kind of simulation.
I would really recommend the use of canvas. I suspect that SVG is usefully supported by fewer browsers than canvas (particularly on mobile platforms, where you can't use any kind of plugin to fix it).
It is possible to do many things using HTML and css e.g. image scaling, and perhaps rotation (depending on CSS version).
I really prefer canvas to CSS, because there are a lot fewer differences between browsers, and the feature set and behaviour is much better defined. Plus, you can't draw shapes very easily (or maybe, at all) using CSS. Drawing shapes using CSS is a big horrible hack if it's even possible.