The World Wide Web Consortium (W3C) has created the Open Web Platform that provides the technical specifications and guidelines for web design and applications to build and render web pages and applications.
Whew - that's a mouthful. The thing to remember is that the web has its own language and the "rule book" or "principals of design" for the web are provided by W3C. The W3C provides these guidelines and tools free to all with the hope of enabling everyone in the world to benefit from the web. This mission is supported worldwide by member companies like Apple, Microsoft, Adobe, Cisco, Intel, Samsung... (if you're curious, visit W3c to check out its membership roll).
W3C is responsible for creating the standards for HTML (Hypertext Markup Language) and CSS (Cascading Style Sheets) that provide the basic building blocks for all web languages, pages and applications. HTML provides structure and CSS provides style. W3C provides an outstanding free tool with easy to understand examples of HTML5 and CSS working together as well as tools to learn the most widely used web languages.