The latest version of CSS Backgrounds and Borders Module Level 3 has changed definition of <bg-layer> and <final-bg-layer> to use <bg-position> instead of <position>. I did a little research into the difference and found that CSS Values and Units Module Level 4 has its own syntax of <position> with notes:

Note: The background-position property also accepts a three-value syntax. This has been disallowed generically because it creates parsing ambiguities when combined with other length or percentage components in a property value.

It's good to know. But there are a couple of problems:

• CSS Values and Units Module Level 4 (WD) – doesn't reflect changes about <position> in Changes section
• CSS Values and Units Module Level 3 (CR) – still refers to CSS Backgrounds and Borders Module Level 3 with no notes about the difference

I believe it should fixed in one way or another to be less confused: add a notes to <position> section and an entry to Changes section to L3, or add an entry to Changes section in L4.