How to support both HTML4 and HTML5?

So I have a complete website written in HTML4 with CSS2/开发者_StackOverflow中文版CSS3 support - i.e. mainly taking advantages of most of the CSS3 where supported.

I want to upgrade the site to support HTML5 - but little confused [using Visual Studio 2010 / ASP.NET MVC etc] how I can support both HTML4 and HTML5 ?

Do I need to rewrite the entire site in HTML5 or ? Little confused how to support both HTML4 and HTML5 ?

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You don't need to do anything.

HTML5 is an umbrella term for a collection of loosely-related client-side features supported by recent browsers.
You can just use any features you like.

Just make sure to implement a fallback so that the site still works in browsers that don't support the new features.
The Modernizr library can be very useful here.

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You can use the new HTML5 tags even in HTML4.
However, you'll need to include a shiv to make them style-able in IE < 9.
Modernizr already includes this.

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What HTML5-specific elements do you need on your page? Are you using things like audio, video, canvas, etc. elements? If you're not, then a HTML4-compliant site should be HTML5-compliant as well.

By the way, you can now get HTML5 Intellisense for Visual Studio 2010 and 2008.