How to conditionally load CSS

Question 1

To target not IE with conditional comments.

<!--[if !IE]> -->
According to the conditional comment this is not IE 5-9
<!-- <![endif]-->

And you can do even more advanced stuff: http://www.impressivewebs.com/conditional-comments/

Question 2

You can use something like this:

<!--[if lt IE 7]>      <html class="ie6"> <![endif]-->
<!--[if IE 7]>         <html class="ie7"> <![endif]-->
<!--[if IE 8]>         <html class="ie8"> <![endif]-->
<!--[if gt IE 8]><!--> <html>         <!--<![endif]-->

And then target your selectors by:

 .ie8 .your-selector

As described here.

If you want to have a switch you can also at a class to the html-tag, e.g.

<!--[if gt IE 8]><!--> <html class ="i-am-so-happy-it-is-no-ancient-ie"><!--<![endif]-->

and use it like this:

.i-am-so-happy-it-is-no-ancient-ie .your-selector{...}
.ie6 .your-selector {...}

Edit I forgot IE 9.. Acutally Paul Irish suggests to use this form (slightly adjusted by me to make a non ie-switch possible using a single .ie .class - although you have to be aware that ie 6 doesn't support multiple classes, but you get the idea):

<!--[if lt IE 7 ]> <html class="ie6 ie"> <![endif]-->
<!--[if IE 7 ]>    <html class="ie7 ie"> <![endif]-->
<!--[if IE 8 ]>    <html class="ie8 ie"> <![endif]-->
<!--[if IE 9 ]>    <html class="ie9 ie"> <![endif]-->
<!--[if (gt IE 9)|!(IE)]><!--> <html class="no-ie"> <!--<![endif]-->

For an explanation why, see the link above.

Question 3

you can add the CSS conditional statement for IE browser in HTML page only not in the CSS file..

Target IE 8 and HIGHER

<!--[if gt IE 7]>
    <link rel="stylesheet" type="text/css" href="ie8-and-up.css" />
<![endif]-->

<!--[if gte IE 8]>
    <link rel="stylesheet" type="text/css" href="ie8-and-up.css" />
<![endif]-->