Simple: Intentionally do not have much graphic detail, since they are used at a small scale.
Consistent: Styling within our icon set is consistent and supports a cohesive brand.
Recognizable: Fast to recognize and clearly show relation to the action, item or idea they are representing.
Accessibility Requirements & Resources
We strive to meet the same contrast rules in icons as we do in typography.
If the icon has descriptive text next to it, or if the icon is only decorative, then you don't need to include an alt tag. If the icon stands alone and is giving information to the user, an alt tag is required.
Try to make icons visibile to all users. For example, icons should be no smaller than 14 x 14 px.
Icons
Spectrum Enterprise Icon Library
Principles
Accessibility Requirements & Resources
Mountaineer Design System Link