HTML 5. In two or three years time I'm willing to bet a large portion (if not a majority) of desktop applications on all platforms will be written using HTML 5 technologies - in particular offline storage and web workers. If you're an expert with CSS, JavaScript and the various HTML 5 APIs you'll be in a very good position.
The barrier to learning HTML 5 for the entrenched HTML/CSS/JS talent is too low and the migration path is too clear that being an "HTML5" expert will be notable enough to provide exclusive niche marketability.