What Does the `\9` in a CSS `width` Property Declaration Actually Do?

CSS Hack Exposed: Demystifying the Meaning of 9 in Width Property Declaration

In the realm of CSS, the 9 suffix can raise eyebrows. Here's a closer look at its purpose and the implications it has in web development.

Understanding 9: A Browser-Specific CSS Hack

At a glance, 9 seems like a peculiar addition to a CSS width property declaration, such as:

width: 500px\9;

Unveiling its true nature, 9 is actually an obscure "CSS hack" exclusive to Internet Explorer versions 7, 8, and 9. This means that the presence of 9 restricts the application of that specific CSS rule solely to these older IE browsers.

Breaking Down the CSS Declaration

In the above example, "width: 500px9;" implies that a width of 500 pixels will take effect only when the page is rendered in IE 7, 8, or 9. All other modern browsers, including future versions of IE, will simply disregard this line, leaving the element's width unaffected.

Practical Applications

This CSS hack can be useful in scenarios where you need to apply different styles to an element in IE compared to other browsers. For instance, you could have:

#myElement {
    width: 300px;
    width: 500px\9;
}

As a result, "#myElement" would be 500 pixels wide in IE 7-9, while in other browsers, it would have a width of 300 pixels.

Noteworthy Exceptions

It's worth noting that browsers have evolved significantly since 2011 when the original answer to this question was provided. While 9 was primarily associated with IE 7-9, it has also been observed to function in IE 10. However, this hack is generally considered outdated and should be avoided in modern web development practices.

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