Codecademy is a great resource for those wanting to learn HTML and CSS. There are lessons and courses you can take to learn and improve your skills. Step by step, it guides you through principles to help you fully understand the basics.
W3Schools is a very trusted site to find information on HTML and CSS coding. They explain the principles simply and have everything you need to know on the subject. When you have any questions about a particular principle, visit this site as it will be a big help.
CSSdesk allows you to make live changes to code so you can see the changes in real-time. The interface is simple and easy to follow. This way you can see immediately how the code you are wanting to implement will look.