Is the latest version of Internet Explorer now fully HTML5 & CSS3 compatible?
Also i want to know if many people are still using older versions of Internet Explorer like IE6-IE8.
Thanks.
No browser is fully HTML5 and CSS3 compatible because neither of those standards is finished yet and so are still subject to change.
The latest version of IE is as compatible as other browsers.
IE7 and earlier are effectively dead as almost no one uses them any more. IE9 is also close to being dead. The most popular problem browser is now IE8 which still has too many people using it to be able to completely ignore.
It appears that roughly 6% of desktop users are still on IE8:
As felgall said, IE11 is essentially as up with HTML5 and CSS3 as the other modern browsers (ahead on some things, behind on others, which is the case with all of them). Check out the caniuse site for info on who supports what. http://caniuse.com/
As you can see IE is far far behind all other browswers !Those numbers don’t mean anything as they don’t tell you what parts of the proposed HTML 5 they refer to. Also HTML 5 might have changed since those numbers were produced and so browsers may or may not be supporting as much of HTML 5 as that chart suggests.
If the parts IE11 supports all end up in the standard and some of those are not supported by the others then IE11 might be the closest to supporting the final standard.
Anyway you can expect Microsoft to be cautious about supporting proposals before they become standard. They were the first to implement CSS 2 and a couple of significant things changed in between their implementation and CSS 2 becoming a standard resulting in quirks mode.
All that chart shows you is the likelihood of the browsers ending up with a new quirks mode through having implemented parts of HTML 5 that are still subject to change. Actually looking at how much has been implemented will only make sense once HTML 5 is actually finished.
That page fails to indicate what the numbers refer to. People have forgotten how to present information meaningfully. 
I really appreciate all your comments.
Thank you very much.
I love this forum.