PHP is a growing skill which I build in regularly. I've worked with a number of frameworks including Wordpress and codeigniter to custom builds. Admittedly, I have a huge preference towards being able to bring things to life, so back-end technologies play second fiddle to that. Where I can tie them all together – that’s the space I enjoy.
With new concepts, ideas and tools entering the space all the time, HTML and CSS are not to be underestimated. The latest HTML-5 and CSS3 technologies combined with a whole host of new abilities such as appcache, local database, history, canvas, and 3D means there is now a goldmine of tools; perfect for making the latest and greatest web apps and building superb sites designed for users no matter what device they are on.