HTML & CSS
What we understand today as HTML5 has had a relatively turbulent history. You probably already know that HTML is the predominant markup language used to describe content, or data, on the World Wide Web. HTML5 is the latest iteration of that markup language, and includes new features, improvements to existing features, and scripting-based APIs.
That said, HTML5 is not a reformulation of previous versions of the language — it includes all valid elements from both HTML4 and XHTML 1.0. Furthermore, it’s been designed with some primary principles in mind to ensure it works on just about every platform, is compatible with older browsers, and handles errors gracefully. A summary of the design principles that guided the creation of HTML5 can be found on the W3C’s HTML Design Principles page.
First and foremost, HTML5 includes redefinitions of existing markup elements, and new elements that allow web designers to be more expressive in the semantics of their markup. Why litter your page with divs when you can have articles, sections, headers, footers, and more?