HTML5 is the newest evolution of the Hyper Text Mark-up language which is the programming structure used by almost every website on the web. Its use is important in many ways, especially for the future of web semantics and Search Engine Optimization, which will make your site more noticeable on the web. Unlike many mojoPortal skins, Swift Blue is HTML5 compliant.
Check out this article for more info: What is HTML5 anyways?
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