[css3-values] Status of attr()

Hello,

Looking over the CSS 3 Values specification [1], I noticed that the attr, calc, and toggle features were listed as at-risk. I am particularly interested in the attr feature, and was wondering what at-risk meant in this case. Were there implementation or browser support concerns? Would it be moved to the level 4 specification if it did not make it into level 3? I was interested in investigating implementations, but wanted to see what the current concerns were with the feature.

-Bear