The background-color: color\9; hack

Last updated 2016-01-20.

The hack

.your-selector-here {
  background-color: somecolor\9;
}

…where somecolor is a CSS color value.

Source: Browser CSS Hacks by Paul Irish

Important note! This article only applies to the background-color property and only applies when there is no space between somecolor and \9. Adding spaces or using different properties changes the compatibility of the hack.
You have been warned.

Live testcases

If the rectangle above is light green, then your browser supports this hack for color keyword values (e.g. lime).

If the rectangle above is light green, then your browser supports this hack for RGB colors in hexadecimal notation (e.g. #adff2f).

If the rectangle above shows the word "Affected" on a green background, then your browser supports this hack for the transparent keyword color value.

If the rectangle above is light green, then your browser supports this hack for RGB colors in functional notation (e.g. rgb(173, 255, 47)).

How it works

Backslash (\) is used as an escape character in CSS. So \9 is an escape sequence which encodes the tab character (U+0009). Presumably the affected browsers are interpreting it like a literal tab character and thus ignoring it when parsing background-color values, since whitespace between the value and the semicolon that ends a declaration doesn't matter in CSS (something like background-color: red   ; is perfectly valid).

The hack seems to work for colors represented using any syntax in IE8. It seems to work for colors represented using any syntax except functional notation (e.g. rgb(173, 255, 47)) in IE9-10 (see Note 2).

The hack doesn't seem to work in IE11 or any other browser.

Affected browsers

Check out the links for corroborating screenshots.

Test results

Chrome	OS X Safari	iOS Safari	Firefox	MS Edge1	Internet Explorer
49 Unknown	10 Unknown	9.3 Unknown	46 Unknown	13 Unknown
48 Unaffected	9.0.2 Unaffected	9.2 Unaffected	45.0a2 Unaffected	12.10240 Unaffected	11 Unaffected
47 Unaffected	8.0.8 Unaffected	8.4 Unaffected	44.0b7 Unaffected	12.10166 Unknown	10 Affected2
26 Unaffected	7.1.7 Unaffected	7.1 Unaffected	43.0.4 Unaffected		9 Affected2
25 Unknown	6.2.7 Unknown	6.1 Unknown	4.0.1 Unaffected		8 Affected
					7 Unknown