I'm worried about
background-position-y. Gecko (Firefox) and Presto (Opera) don't support them, but Webkit (Chrome, Safari) does...
Does anyone know (with official references) if this is (or will be) included in the standard?
-y was proposed for CSS 3 but it got rejected as the working group considered the use case too weak to introduce new properties for. Additionally, there seemed to be some ambiguity concerns with multiple background images and CSSOM, the API behind CSS. Im not sure how WebKit and Trident did implement them and how they addressed these concerns.
The CSS WG blob has just reported yesterday that there will be no change to
transform-origin syntaxes. The reason is probably due to not getting any further delays.
Maybe it will get added to CSS 4. There is a current thread on the [email protected] mailing list that you might find interesting.
Update So apparently these properties have been approved for CSS 4 (see [CSSWG] Minutes Telecon 2014-04-16):
-yapproved for level 4 of backgrounds and borders.
background-position-y are now part of the level 4 of Backgrounds and Borders standard.
RESOLVED: background-position-x/-y, background-repeat-x/-y approved for level 4 of backgrounds and borders.
I guess in some sense you've already answered your own question. No, both background-position-x and background-position-y are nonstandard.