I would like to echo the call made by Garret Murphey and others for the ability to specify whether borders applied externally or internally to an element.
This would not only be a valuable design choice to make available, but it would do wonders for developer tools like Firefox’s Web Developer module that impose borders on elements to highlight them for debugging. Currently, such borders often break the layout of a closely packed page, rendering them useless. Internally applied borders would have no effect on layout and so would be desirable in this and many other cases.
Also, since I have the conch:
+ Rounded corners: Awesome. Per-corner Radii: Awesomer.
+ Gradients: I think basic gradient functionality would be a wonderful bandwidth-saver and well within the scope of what CSS should be for. I would even love to see gradients applicable to borders!
+ Defined constants ala Nathan Kitchen’s comments would be huge. Being able to assign attributes a pre-defined variable instead of a named constant each time would both increase functionality and readability/ease-of-design in a potentially elegant way. One question: Can my javascript change these variable values in the DOM, and if so is it reasonable to require browsers to propogate the change throughout the rendered page?