What is HTML5?
HTML5 is the next big thing in the web design world. The main difference with HTML5 compared to its predecessor, HTML4, is the presence of semantic markup. In previous versions of HTML, specific tags such as <div> (for a block element), <span> (for an inline element), <footer> (for an element in the footer section of a page), etc. were used. In HTML5, you'll be able to use markup such as <item rdf:about="http://example.com">Example Item</item> to define arbitrary elements and their relationships.
RDF, OWL, and XML
The Resource Description Framework (RDF), Web Ontology Language (OWL), and Extensible Markup Language (XML) are used to semantically describe elements, ie. arbitrary things such as people, events, car parts, etc. HTML on the other hand, was limited to describing documents and their relationships (links) to other documents. While XHTML attempted to make more use of semantic markup, HTML5 plans on taking full advantage of these object-oriented attributes.
How Long Before We See HTML5?
It has been ages since the HTML (HyperText Markup Language) specification and technology has been upgraded, and it appears it will be ages before HTML5 is fully implemented and used in the broader web market. In fact, the editor of HTML5, Ian Hickson from Google, doesn't expect HTML5 to reach the "W3C Candidate Recommendation Stage" until 2012 (the original planned date was the end of 2010, and W3C Recommendation (official implementation) until 2022!
HTML5 to Replace Proprietary Technologies (ie. Flash)
Nevertheless, as with past HTML releases, many parts of the HTML5 specification, particularly those that will replace proprietary plug-in technologies such as Adobe Flash, Microsoft Silverlight, and Sun JavaFX, are stable and are in the process of being implemented in products. The <audio> and <video> tags, for example, will provide representative technologies through a new interface, and will allow audio and video elements to be rendered by browsers via standard HTML technologies.
Like the industry standard for all web design coding, with the new HTML5 comes the need for an HTML5 Validator. While there are plenty of third party validators out there, you're best off using the official W3C (World Wide Web Consortium) engine at validator.w3.org. Most third party validators either use this engine or are based off it. But since the W3C ultimately determines the standards and specifications of HTML5, you're best off going to the source.
As you'll see, you can validate by URI (web address), File Upload (upload your HTML5 file), or Direct Input (copy and paste your HTML5 code). You'll see that an HTML5 (experimental) validation option is available already. By using this HTML5 validator on a regular basis as you begin to adopt elements of the HTML5 standard in your projects, you'll be kept abreast of the technology as it evolves and matures, and will be ready to implement it in full once it reaches its final release/ acceptance stage.