The length-property: length \9; hack

Last updated 2016-02-03.

The hack

.your-selector-here {
  some-length-property-here: somelength \9;
}

…where somelength is a CSS length value and some-length-property-here is a CSS property whose value can be a CSS length.

Source: Browser CSS Hacks by Paul Irish

Important note! This article only applies to the specific properties mentioned below and only applies when there is space between somelength and \9. Adding spaces or using different properties may change the compatibility of the hack.
You have been warned.

Live testcases

Affected
Unaffected

If the square above shows the word "Affected", then your browser supports this hack for the width and height CSS properties.

Affected
Unaffected

If the square above shows the word "Affected", then your browser supports this hack for the margin-{top,bottom,left,right} CSS properties.

If the above square is light green, then your browser supports this hack for the padding-{top,bottom,left,right} CSS properties.

If the above square is light green, then your browser supports this hack for the border-{top,bottom,left,right}-width CSS properties.

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 length values, since whitespace between the value and the semicolon that ends a declaration doesn't matter in CSS (something like width: 5px   ; is perfectly valid). The space before the \9 doesn't appear to affect its compatibility. IE8-10 are known to suffer from the bug that this hack relies upon.

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 		45
Unknown 		13
Unknown
48
Unaffected 		9.0.3
Unaffected 		9.2
Unaffected 		44.0
Unaffected 		12.10240
Unaffected 		11
Unaffected
47
Unaffected 		8.0.8
Unaffected 		8.4
Unaffected 		43.0.1
Unaffected 		12.10166
Unknown 		10
Affected
26
Unaffected 		7.1.7
Unaffected 		7.1
Unaffected 		4.0.1
Unaffected 		9
Affected
25
Unknown 		6.2.7
Unknown 		6.1
Unknown 		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.