w3Schools is a simple but excellent website for learning how web design works including the basics–HTML and CSS–and more advanced topics like Javascript and other web-based programming languages. The links below are great places to start your understanding of HTML and CSS.
This PDF document, HTML Basics and Tags, explains the basic structure of an HTML file and describes some of the most common HTML tags.
This CSS Cheat Sheet is a printable PDF file that contains the basic CSS rules that can be applied to HTML content.