A standard for user interface icon design
Editing this article to discuss general icon design standards, which I believe are needed. Icons should down-rez gracefully to small sizes, with vertical and horizontal edges aligning to the pixel grid at sizes as small as 9 pixels. A 3×3 sub grid should be adhered to when designing icons.
As you can see in the last icon, the 3×3 subgrid isn’t honored and some edges get blurry: