Are `background-position-x` and `background-position-y` Officially Part of the W3C CSS Standard?

Is CSS background-position-x and background-position-y a W3C Standard?

Question:

There is concern surrounding the use of background-position-x and background-position-y properties in Gecko (Firefox) and Presto (Opera) engines compared to their support in Webkit (Chrome and Safari). Is there any official information regarding their inclusion or exclusion from the W3C CSS standard?

Answer:

The proposal to split background-position into background-position-x and background-position-y for CSS 3 was rejected due to perceived weak use cases and ambiguity concerns related to multiple background images and the CSS Object Model (CSSOM).

WebKit and Trident engineers implemented these properties, despite the lack of standardization, but the specific implementation details are unknown.

In a recent CSS WG announcement, it was confirmed that there would be no changes to the syntax of background-position or transform-origin. This decision suggests that the proposed changes may not be included in future CSS versions.

However, it is possible that these properties may be added to CSS 4. A thread on the CSSWG mailing list discusses this topic further.

Update:

In April 2014, the CSSWG approved the inclusion of background-position-x and background-position-y in CSS 4, as noted in their meeting minutes:

"RESOLVED: background-position-x/-y, background-repeat-x/-y approved for level 4 of backgrounds and borders."

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