Opera 9.5, according to this Opera Dev article, and also this MDC article has support for the CSS 3 background-size rule through -o-background-size vendor extension. I've been playing around today with the code from the MDC article and have got it to work in Firefox and Chromium, but it doesn't work in Opera 10 Beta 2:
body {
background-attachment: fixed;
background-image: url('bg-image-1.jpg');
background-size: 100% 100%;
-moz-background-size: 100% 100%; /* Gecko 1.9.2 (Firefox 3.6) */
-o-background-size: 100% 100%; /* Opera 9.5 */
-webkit-background-size: 100% 100%; /* Safari 3.0 */
-khtml-background-size: 100% 100%; /* Konqueror 3.5.4 */
-moz-border-image: url('bg-image-1.jpg') 0;/* Gecko 1.9.1 (Firefox 3.5) */
}
In fact, Opera's own example doesn't seem to work in Opera 10 Beta 2, which leads me to believe support has somehow been removed, but there's no mention of it in the beta 1 or beta 2 changelogs.
Can someone who still has Opera 9.5 installed check the the Opera example works (should see green bars at the sides of the page)? And if anyone's seen any documentation for changes in background-size support between 9.5 and 10 beta please post a link.
Here's what it looks like in the Linux build of 10b2:



background-sizeis working for me in Opera 11.10, although the Opera example I linked to doesn't work because that only lists-o-background-size– robertc May 9 '11 at 16:15