HTML and XHTML both impose rules on which elements can be contained within which other elements. The definitive guide to these rules are the various DTDs (Document Type Definitions) that govern the different flavours of HTML. Unfortunately, DTDs aren’t exactly friendly to read. Enter Cameron Adams’ brilliant DTD Mapper, which presents an easily navigated tree style view of a DTD and makes it easy to see which elements are allowed where. Cameron’s blog entry has more.
Plenty has been written about the CSS Box Model (I made my own attempt at explaining it last year) but it’s rare for anyone to discuss the way in which the layers of the box model interact in the third dimension. Jon Hicks has published a beautiful diagram of the 3D CSS Box Model which neatly demonstrates this little documented aspect of CSS.
I’m pretty terrible at picking colour schemes, but thankfully the web is overflowing with tools to make it easier. My favourite is still Eric Meyer’s Color Blender which calculates a number of mid points between two colours, but Pixy’s impressive Color Scheme is very useful as well. Dave Shea’s recent article on Colour Schemes has some good tips, and Andy Clarke has some useful tips on creating a consistent palette using Macromedia Fireworks (though his technique could easily be ported to other applications).