HTML5 is the newest standard for HTML development, it has a range of additional features that will change the way we interact with the web. This is great for web development, there is a problem though, older browser versions specifically IE 6, 7 and 8 do not support the new HTML5 elements so that you cannot attach CSS to those elements.
The first is to wrap your HTML5 element within an additional div, this div then takes the styling the new HTML5 element would have had. The benefit of this is that you get your semantic HTML5 element but at the price of an additional tag for each new element, effectively watering down the semantic meaning and increasing the code density on the page.
<div id="navigation"> <nav> <ul> <li><a href="#"></a></li> </ul> </nav> </div>
Another option is to serve different code using conditional comments. The benefit is that you have control over what is rendered by which browsers, the price though will be a large number of conditional comments littered throughout your code creating an additional weight of download and increasing the render time.
<!--[if lt IE 9]><div id="navigation"><![endif]--> <!--[if IE 9]><nav id="navigation"><![endif]--> <!--[if !IE]><nav id="navigation"><![endif]-->
After discussing this with @willhowat an alternative method would be to remove the new elements entirely and reference them through a reserved classname. It’s benefit is to remove any additional tags used to structure the page and by attaching the classname the element can be programmatically referenced. Of course the price is you initially lose access to these semantic elements.
<div id="navigation" class="navigation"> <ul> <li><a href="#"></a></li> </ul> </div>
By referencing the classname it would be possible to switch HTML elements server side, having the same effect as the conditional comments but without the inherent price. Failing that when the older versions of IE finally fades away it would be possible to manually swap out the code.
<nav id="navigation"> <ul> <li><a href="#"></a></li> </ul> </nav>