The name "HTML5" is typically used by most people at the moment to describe all the exciting new features in web browsers. Not all of these features are actually part of the HTML5 specification, for example, CSS and Javascript have new features that are mentioned under the HTML5 banner. I'm going to assume that you mean all these functions.
My answer will be to figure out what features you can implement, but stay backward compatible. Many of the features of HTML5, CSS3, etc. They can be added to your site without making it unsuitable for older browsers.
Some examples: (but spend more time researching)
New input types such as <input type='date'> and <input type='number'>
They give you new features in a modern browser, but older browsers will still display a normal input field. Additional information: http://www.456bereastreet.com/archive/201004/html5_input_types/
HTML5 semantic tags such as <section> , <footer> , etc.
This gives you extra meaning for your HTML tags, which is good for SEO and other automated systems that look at your site. They are generally backward compatible, although you need to enable an IE hack such as HTML5Shiv to support them in IE6 / 7/8. However, I would suggest using Modernizr , which includes HTML5Shiv plus a stack of other useful features.
Some CSS3 features, such as border-radius . IE is not aware of this, but as a result, IE will have square corners, and other browsers will have rounded corners. Thus, nothing that breaks the site for IE users.
But if you really want it, IE can be created to support a number of CSS3 features, including border-radius , using tools like CSS3Pie .
Instead of directly using SVG graphics, use a Javascript graphics library such as Raphael , which will draw SVG in browsers to support it and return to VML for IE (which will even work in IE6).
Obviously, things like the <video> or <canvas> tags will not work in older browsers, but to be honest, you will need a lot of time just by using functions that you can use right now; by the time you are ready to take another step, it will still move anyway.
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