@Sven Groot: Great points. Indeed, web programming has grown up and you can build very sophisticated apps with HTML, CSS, and JS. So many libraries, so little time. Browsers have also evolved greatly, enabling the level of app development we're all seeing and/or doing. With the emergence of technologies like WebGL, now you can natively (as in native to the web platform, so JS...) leverage hardware that was off limits not too long ago... Very interesting things are being done in CSS, too.
As always, use the tool that's designed to be used to solve the problem(s) at hand. For building cross platform (browser) web apps, HTML, CSS, and JS are the right tools.