HTML5 is becoming more and more popular, with this has some issues. If you want to support some of the older browsers which are still being used such as IE8 and lower then using HTML5 you are going to have some trouble.
Internet Explorer 8 And Lower
The problem with IE8 and lower browsers is that they were created years ago before any of the new HTML5 tags were invented so it can't support them. Microsoft will not release updates for these browsers because they have created IE9 so they want people to upgrade to this.
If people don't upgrade then they are stuck with an out of date browser which can't support these new HTML tags.
The browsers won't know anything about these new tags of article, section, hav, header, footer so it can't support them. If you are using these tags you are most likely be styling the tag itself or you would have to add a class to the tag, which takes away the point of using HTML5.
As the browser doesn't know about these tags then it can't style them so you will find that in IE8 or lower your websites will be without many styles.
Correct Way To use HTML5 For IE
<script> document.createElement('header'); document.createElement('section'); document.createElement('article'); document.createElement('aside'); document.createElement('nav'); document.createElement('footer'); </script>
But this is not a good way of working as you will have to remember to include this at the top of every page of your website and then if you want to add a new HTML5 element you have to go back to every page to add this in.
Copy the following at the top of your pages and it will include this code file on your website if the visitor is using an IE browser lower than version 9.
<!--[if lt IE 9]> <script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script> <![endif]-->