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

Color keyword

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

Hex color

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

Affected

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

rgb(R,G,B)

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
Microsoft Edge
Internet Explorer
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
Note 1: MS Edge versions refer to the EdgeHTML rendering engine version, not the Edge app version shown in Edge's "About" screen. See this Can I Use issue.
Note 2: Functional color notation (e.g. rgb(173, 255, 47)) is not compatible with this hack.