Apply style ONLY on IE

@media screen and (-ms-high-contrast: active), (-ms-high-contrast: none) {
   #myElement {
        /* Enter your style code */
   }
}

Explanation: It is a Microsoft-specific media query. Using -ms-high-contrast property specific to Microsoft IE, it will only be parsed in Internet Explorer 10 or greater. I have used both the valid values of the media query, so it will be parsed by IE only, whether the user has high contrast enabled or not.

Update 2017

Depending on the environment, conditional comments have been officially deprecated and removed in IE10+.

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Original

The simplest way is probably to use an Internet Explorer conditional comment in your HTML:

<!--[if IE]>
<style>
    .actual-form table {
         width: 100%;
    }
</style>
<![endif]-->

There are numerous hacks (e.g. the underscore hack) you can use that will allow you to target only IE within your stylesheet, but it gets very messy if you want to target all versions of IE on all platforms.

Apart from the IE conditional comments, this is an updated list on how to target IE6 to IE10.

See specific CSS & JS hacks beyond IE.

/***** Attribute Hacks ******/

/* IE6 */
#once { _color: blue }

/* IE6, IE7 */
#doce { *color: blue; /* or #color: blue */ }

/* Everything but IE6 */
#diecisiete { color/**/: blue }

/* IE6, IE7, IE8, but also IE9 in some cases :( */
#diecinueve { color: blue\9; }

/* IE7, IE8 */
#veinte { color/*\**/: blue\9; }

/* IE6, IE7 -- acts as an !important */
#veintesiete { color: blue !ie; } /* string after ! can be anything */

/* IE8, IE9 */
#anotherone  {color: blue\0/;} /* must go at the END of all rules */

/* IE9, IE10, IE11 */
@media screen and (min-width:0\0) {
    #veintidos { color: red}
}


/***** Selector Hacks ******/

/* IE6 and below */
* html #uno  { color: red }

/* IE7 */
*:first-child+html #dos { color: red }

/* IE8 (Everything but IE 6,7) */
html>/**/body #cuatro { color: red }

/* Everything but IE6-8 */
:root *> #quince { color: red  }

/* IE7 */
*+html #dieciocho {  color: red }

/* IE 10+ */
@media screen and (-ms-high-contrast: active), (-ms-high-contrast: none) {
   #veintiun { color: red; }
}