HTML5

HTML5 is the fifth revision of the HTML standard. With the addition of many new APIs, including better media support, document editing, drag and drop, etc., HTML5 has been touted as a powerful web development platform. I think it's probably more accurate to see HTML5 as a highly capable presentation layer (in conjunction with CSS); almost certainly you'll still need to write some application logic on the client side, and the logical choice there is JavaScript. 

For HTML5 development, then, you're looking at a minimum of three technologies:

• HTML5
• CSS
• JavaScript

plus whatever tooling you may need on the server side (if present). There your choices are almost as varied as software development itself.