On the standards track: Yes
Is Experimental: No
Is Deprecated: No
In Chrome version 3 on 2009-09-15 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In Chrome Android version 18 on 2012-06-27 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In Edge version 12 on 2015-07-29
In Firefox version 4 on 2011-03-22
In Firefox for Android version 4 on 2011-03-29
In Internet Explorer version 9 on 2011-03-14
In Quest Browser version 5.0 on undefined Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In Opera version 10 on 2009-09-01 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords. Opera 9.5's computation of the background positioning area is incorrect for fixed backgrounds. Opera 9.5 also interprets the two-value form as a horizontal scaling factor and, from appearances, a vertical clipping dimension. This has been fixed in Opera 10.
In Opera Android version 10.1 on 2010-11-09 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords. Opera 9.5's computation of the background positioning area is incorrect for fixed backgrounds. Opera 9.5 also interprets the two-value form as a horizontal scaling factor and, from appearances, a vertical clipping dimension. This has been fixed in Opera 10.
In Safari version 5 on 2010-06-07 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In Safari on iOS version 4.2 on 2010-11-22 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In Samsung Internet version 1.0 on 2013-04-27 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In WebView Android version 4.4 on 2013-12-09 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.
In WebView on iOS version 4.2 on 2010-11-22 Note: WebKit-based browsers originally implemented an older draft of CSS3 background-size in which an omitted second value is treated as duplicating the first value; this draft does not include the contain or cover keywords.