HTML5 is the new version that came after HTML4.01. HTML4.01 was released in 1999 and since then Web has been changed to very large extent. To embrace the changes of new web, HTML5 came as the new standard. HTML5 is a joint venture of W3C and WHATWG to standardize the web and be supported by all browsers.
Design principles of HTML5
You can read the official HTML5 Design Principles here. I have made gist out of that document and summarized a few of the important points below. HTML5's design principles are:
- Support of existing content. For example: a document written in a previous version of HTML should be rendered exactly the same and can be processed as HTML5 document.
- Unbroken Error handling. For example: the way existing HTML is handling broken tags, HTML 5 should handle the errors in the same way.
- Design features should be accessible to users with disabilities. For example: a blind user cannot see an Image. So with the img tag there should be a provision for a non-visual alternative to text.
- Any feature should be independent of the devices and media.
- If there is already a popular technology being used for a particular purpose, use that, rather than creating a new technology for the same purpose.
- If a particular practice is already popular, use that, rather than obsoleting that and forcing developers to adhere to a new practice. For example, the (somewhat odd) <br/> tag.
- Support classical and existing HTML syntax.
- Ensure that any new feature will adhere to high web security standards.
- Features should provide simple solutions over complex solutions.
- Error handling should be be able to address interoperable implementation.
- There should be more markup, and markup should replace scripts as much as possible.
- For media usage, external plugins should be minimal.