Detect support for CSS property with CSS only - iphone

Question

Short question
Using CSS only, how do I detect that the background-size property is supported? If it's not supported, I would like to provide some fallback CSS. I already know how to do this with Javascript, but it's cleaner with CSS.
Long question
I have a high resolution sprite image that needs to look good on all cell phones, regardless of its exact pixel density. By using a background-size trick, I can scale the sprite appropriately.
.sprite {
background-image: url(sprite180x76.png);
/* 180 / 2 = 90 */
background-size: 90px auto;
}
There are some iOS and Android versions that don't support the background-size property, so the sprite would look twice as big as it should be. For these old systems, I would like to load up a low resolution sprite with no background scaling:
/* fake CSS */
#notSupported(background-size)
.sprite {
background-image: url(sprite90x38.png);
}
}

Answer 1

CSS doesn't have conditional statements as it's not a programming language like Javascript. Although, I believe there might be some kind of conditional statement in the works for CSS.
You'll have to rely on JavaScript to achieve any kind of conditional test case for CSS.
Meaning, you can't directly detect support for CSS.
However, CSS does have a "trick" thanks to its "Cascading" nature, but it's only usable when looking to replace some older code with newer code for the same style.
That sounds funny, here's a couple of examples:
-moz-border-radius: 6px;
-o-border-radius: 6px;
-webkit-border-radius: 6px;
-ms-border-radius: 6px;
border-radius: 6px;
In browsers that do support the official CSS, it will the style as denoted on line 5. While in older versions of say Firefox, line 1 will get applied and line 2-5 will get ignored because they're unknown.
Another (and perhaps better) example could be:
background-color: #AAA;
background-color: rgba(0, 0, 0, 0.5);
This code will give the background a grey color, while newer browsers will give it a black color with a 50% transparency, overriding the old color.
Hope that helps a little.
Cheers!
-- Update --
I did just come across something that might help. In Aaron Gustafson's book "Adaptive Web Design" he mentions how CSS will ignore an entire rule if a given browser/renderer doesn't support a given selector.
With the concept above, if you can find a selector that was not implemented in the older version but is available in the newer version you could do something like this:
/* fake CSS */
.sprite {
background-image: url(sprite90x38.png);
}
[[ selector that is supported by newer browser/OS ]],
.sprite {
background-image: url(sprite180x76.png);
/* 180 / 2 = 90 */
background-size: 90px auto;
}
The idea is that for the "old" browsers you can load the old PNG but for the newer browser it will load the larger PNG and apply background size.
The only thing I would be concerned about is if this causes the supportive browsers to load both images but apply one.
And this still requires finding an unsupported selector in one version of another. Quicksmode.com might help you find one:
http://www.quirksmode.org/css/contents.html
-- UPDATE 2 --
I put this in the comments but I'll add it here as it might help. I spent some time trying to find out what browser version iOS 3.1.3 supported and therefor what selectors might be possible to use with the above trick.
What I found was this Apple developers site: http://developer.apple.com/library/safari/#documentation/appleapplications/reference/SafariCSSRef/Articles/StandardCSSProperties.html
If you do a page search (ctr+f) for background-size, it shows that iOS 1+ supported a proprietary version called:
-webkit-background-size: length
-webkit-background-size: length_x length_y
That might be a possible solution. If you add that before the real one, you can ensure backwards compatability.
-webkit-background-size: length
background-size: length
Hopefully that helps find alternate solutions since the original question of doing a conditional test to see if a rule is supported is not possible in CSS right now.

Answer 2

You can't (for now) detect support of property A and, given or not this support, serve different values for property B with CSS only ...
... except if the browser support for properties A and B is exactly the same! Then instructions below:
selector {
propertyA: valueA;
propertyB: valueB;
}
will both fail or both succeed.
The remaining problem is to find a CSS property that has the exact same support than background-size :)
I was thinking about multi-background : it should (not) work in IE6/7/8 according to CSS Background Properties support in Standardista but I can't test in iOs and Android, only in bada/Dolfin 2.0 (Samsung Wave; also based on Webkit).
Here's a Fiddle : http://jsfiddle.net/PhilippeVay/2VaWu/
that shows a paragraph with only a simple background that any browser should display and then another paragraph with both a simple background and (*) a multi-background resized with background-size that only modern browsers should display (older browsers should display the same background as for the first paragraph).
Fx9 and dolfin 2.0 both display correctly the second paragraph. IE8 doesn't, as expected.
Another solution would be to use a selector understood by browser versions that also understand background-size but not understood by others. Though it's easier to find for IE than for smartphones based on Webkit!
(*) using a different CSS rule with higher specificity, for the purpose of the demo. In real world, there'd be only one rule with simple background defined before the multiple background.