HTML4と、HTML5の違い

1.4 Impact on Web Architecture

The following areas / features defined in HTML 5 are believed to impact the Web architecture:

• The use of the DOM as a basis for defining the language.
• The concept of browsing contexts.
• The distinction between user agent requirements and authoring requirements.
• The use of imperative definitions rather than abstract definitions with the requirement of black-box equivalence in implementations.
• The new content model concepts (replacing HTML's block and inline concepts).
• The focus on accessibility as a built-in concept for new features (such as the hidden attribute, the progress element, et cetera) instead of an add-on (like the alt attribute).
• The focus on defining the semantics in detail (e.g. the outline algorithm, replacing the vague semantics in HTML4).
• The datagrid element.
• The menu and command elements.
• The origin concept.
• Offline Web application caches.
• The definition of the browsing context "navigation" algorithm and the related session history traversal algorithms.
• The content-type sniffing and character encoding sniffing.
• The very explicit definition of a parser.
• The contentEditable feature and the UndoManager feature.
• The Drag and Drop and Copy and Paste architecture.
• The cross-document messaging feature (the postMessage API).
• The new sandboxing features for iframe.
• The definition of URL.

このページは、HTML5の入口となるドキュメントだと思います。正直、概要をキャッチアップするのも、結構、大変なような気がしています。
最近は、日本語の情報もいろいろ出てきているので、そちらをざっとみてから、このドキュメントを読むとよい気がしました。