This approach does not only apply to Internet Explorer 9; this hack affects also Internet Explorer 8, and 10. These three versions of Internet Explorer doesn't require the same solutions to certain layout problems, and as a result, this approach is likely to create more problems than it solves.
Microsoft has provided a method for applying browser-specific styles via the use of conditional comments. This is the means by which you should do browser-specific alteration is needed — please avoid CSS hacks.
To target Internet Explorer 9, use the following conditional comments:
<!--[if IE 9]>
<link rel="stylesheet" href="ie9fixes.css" />
<![endif]-->
Although the above will likely help you resolve your issue more quickly, the correct approach is to ensure that a workaround is even necessary. Make sure that your markup and CSS are valid. More often than not browser-specific problems are the result of invalid code.
Support for conditional comments was dropped in Internet Explorer 10.