If you want to develop websites or cool web applications, browser games etc HTML (Hyper Text Markup Language) is the first thing you should learn, it’s the language of web. HTML5 has lot of new exciting features, so you can create awesome websites/apps in a much easier way.
CSS (Cascading Style Sheets) is used for styling the html documents/pages. This way, the design and contents do not mix up. CSS3 is the latest version, with a lot of amazing features that allow you to create cool designs in much effective way.
For Learning HTML5/CSS3, these free tutorials/resources are great for beginners –
But you can’t just learn html/css by reading those manuals, then how ? Create a website (may be for some real purpose or as a fun project), apply those skills (whatever you learned, may be just in a couple of hours). you don’t even need a server or something for learning html/css, design and test in your browser (locally).