Using the term "angry" to describe the emotion of an infant before the age of six months may be completely wrong. Infants sometimes use "angry" facial expressions when they are really simply upset, or distressed, their emotion isn't directed at someone or something, they are simply noticing a feeling of distress.
The segment of the article that I found to be extremely interesting discussed the expressions infants use and what they might mean. When an infant furrows their eyebrows, it suggests that they are angry, similarly to any person. But rather than compress their lips like most adults, they move their mouths into a square-like angular shape. When sad, they raise the inner corners of the brow, and turn down the corners of their mouths.
However, this is not completely true for all infants, for infants from different cultures seem to express their emotions differently. Chinese infants appear less emotionally reactive than Japanese and American infants. Chinese infants are slower to smile and slower to cry. The cause of this difference is probably cultural, although it is a difficult thing to test.
Using specific words to describe the emotions of an infant may be jumping the gun. They are not developed enough to have as specific emotions as adults.