<header> - Defines a header for a section or page

Check this with Firefox 3.5+, if you find any prob for any tag, please check with opera latest version then

<article> - Defines an article

This example HTML5 page was taken from online at It's All About Me - kodekeek bloga>

<section> - Defines a section

Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aenean dictum fermentum quam, sit amet consequat sapien accumsan ac. Quisque viverra, enim at placerat sodales, nisl mi venenatis mi, non

<video> - Defines a video

<figure> - Defines a group of media content, and their caption
<audio> - Defines sound content
<source> - tag defines media resources for media elements

<embed> - defines embedded content, such as a plug-in.

<command> - Defines a command button

is this article helpfull? Yes

<dialog> - Defines a dialog (conversation)
Sam
Knock, Knock.
Eric
Who's there?
Sam
Justin.
Eric
Justin who?
Sam
Justin time for dinner!
<meter> - defines a measurement. Used only for measurements with a known minimum and maximum value.

Your score is: 2 out of 10

<datagrid> - defines a list of selectable data. The datagrid is displayed as a tree-list.
  1. (datagrid row 0)
  2. (datagrid row 1)
    1. (datagrid row 1,0)
    2. (datagrid row 1,1)
  3. (datagrid row 2)
<datalist> - defines a list of selectable data.
<details> - defines details of an element, which the user can see, and click to hide.
Should be open

and you should be able to see this para!

there are few issues with this tag, i'm not clear though, here is a reference