CSS style declarations for different browsers

In some large web site HTML code, we often see <!--[if ie 6]><!--[if ie 7]> code, then, what is the use of them?

This code is rarely applied in most front-end designs, but this is a different browser style declaration to achieve a compatible effect, the following provides an example for you to refer to:

<!--[if IE 6]>
<style>
#head_menu_main li a span{line-height:14px}
#head_menu_country Li a{font:12px/21px Arial Font-weight:bold}
#head_menu_place Li a{font:12px/19px Arial;
</style>
<! [endif]-->
<!--[if IE 7]>
<style>
#head_menu_main li a span{line-height:14px}
#head_menu_country Li a{font:12px/21px Arial Font-weight:bold}
#head_menu_place Li a{font:12px/19px Arial;
</style>
<! [endif]-->
<!--[if IE 8]>
<style>
#head_menu_main li a span{line-height:14px}
#head_menu_country Li a{font:12px/23px Arial Font-weight:bold}
#head_menu_place Li a{font:12px/21px Arial;
</style>
<! [endif]-->