I have seen Bootstrap V3.

There is code like:

<!--[if lt IE 9]>
<script src="js/html5shiv/html5shiv.js"></script>
<script src="js/respond/respond.min.js"></script>

1. Should we use modernizr?
How to handle IE 8 as I see many corrupted designs like
<header></header> as some specifications inside CSS3 and HTML5 will not support IE8

Any good references which work in IE8 and use http://getbootstrap.com/