Of course! I've been using HTML5 and CSS3 on my projects for quite some time now.
These are fantastic web technologies that enable you to do so much more with less
effort. Combined with the fact that there are numerous Javascript libraries that let you
detect which features are supported in the user's browser, there is no reason at all
why you shouldn't be using the new standards.
I can't post links, but an excellent JS library for HTML5/CSS3 feature detection is Modernizr.
Also, check out Twitter Bootstrap and HTML5 Boilerplate. It helps you get off the ground with
new projects in minutes.
Additionally, google for caniuse, they've got excellent charts that give you an overview of what
features are supported in which browsers.
Combine the tech with a responsive web design mindset that caters to tablets, smartphones, and
what-have-you, and you're all set to go.
If you've got any questions, don't hesitate to ask.
- T.