Get up-to-date with HTML5
HTML5 has introduced a lot of new functionality to the web. All major web browsers now support HTML5 (although unofficially), and most older browsers are "backwards-compatible", so there really is no sense making a new site with old semantics. If you're concerned about people visiting your site with a non HTML5 browser, don't be. Most major browsers update automatically, and for those that don't (namely Internet Explorer) there are fall-backs available to make them compliant with HTML5 semantics .
HTML5 is fully backwards compatible with HTML / XHTML, all you need to do is update your DOCTYPE to the new HTML5 declaration as shown below, and you're done! Once you have an HTML5 declaration, you can add any new HTML5 attributes to your website straight away. Simple.
There are significant advantages to HTML5, not only do you get more functionality, but also significantly less HTML is required compared to the previous HTML 4.01 standard. This means reduced source file size, and therefore reduced page-loading times. Take a look at the comparison between markups below.
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Example HTML 4.01 document</title>
<link type="text/css" rel="stylesheet" href="/theme/stylesheet.css">
<title>Example HTML 5 document</title>
<link rel="stylesheet" href="/theme/stylesheet.css">
This is an example of a simple web page, both markups shown above are fully valid HTML. The DOCTYPE declaration with the old HTML 4.01 specification required a reference to a DTD file hosted by the W3C. This is no longer required in the HTML5 specification, and so the new DOCTYPE declaration is much shorter. The easiest way to identify the markup language of any website is by looking for the DOCTYPE at the beginning of the page source.
The biggest mistake I find on HTML5 websites is the overuse of new HTML5 elements in place of DIVs. There is absolutely nothing wrong with DIVs, they are very short tags and can be styled any way you like. Avoid using <section> as a styling element; it is not a wrapper, and should not be used as such.
Be careful with HTML5 elements, while they can be a very powerful replacement for HTML 4.01 elements; there are some compliance issues to think about. Internet Explorer version 8 and below do not understand HTML5 elements, and will very likely render them incorrectly. Fortunately this problem has been addressed; thanks to html5shiv.