I have been using a tool for some PluralSight classes and currently a book I’m reading. Normally I just jump into Visual Studio, but I will admit it is a bit over kill for just playing with HTML5/CSS3. The PluralSight courses and Pro HTML5 with Visual Studio 2012 make use of WebMatrix. In fact when I’m on my Windows 8.1 box I often use CodeWriter for the lightweight stuff, but the instructor and writer used WebMatrix so I followed suit.
Not only is WebMatrix quick and lightweight it has intellisense and even the ability to create your site in Azure if you take that option. Like Visual Studio you can launch your site from the tool in the browser of your choice. I have not explored everything WebMatrix can do, but I will definitely be using it for small minor things where Visual Studio would just be over kill. At this time I really don’t see myself using it for real work, just for when I want to play/learn outside a real project. For example I know you can create an MVC project in WebMatrix but I feel more comfortable in Visual Studio for the real projects.
So if you’re looking for something lightweight that feels like Visual Studio I highly suggest Web Matrix.