CSS-Tricks has got to be, hands down, one of the most interesting and useful resources on the web I have ever found. As CSS has evolved, so too have the methods of doing things, and CSS-Tricks keeps you up to date with the latest.
The site also has an ‘almanac’ feature, where you can lookup CSS selectors and properties. They then show you how to properly use the selectors and properties to achieve the results you’re looking for.
CSS-Tricks also has useful snippets for HTML, CSS, PHP, JavaScript, WordPress and others. These snippets are bits of useful code that show you how to achieve certain results. For example, under the WordPress category, there are snippets that show you how you can display a Facebook “like” button, remove the width and height attributes from inserted images, and even using shortcodes in a template.
There are also forums available where you can discuss different methods, ask for help, or just to discuss all things HTML+CSS.
Visit CSS-Tricks at css-tricks.com.