Specifically, Acid2 tests these functions on your browser; Alpha transparency in PNG-format images, which is seen when the eyes of the smiley face use "alpha transparency", which is part of the 1996 Portable Network Graphics specification. Alpha transparency blends the eyebrows into the face smoothly and elegantly. This was a significant issue because Internet Explorer 6, the most widely used web browser at the time Acid2 was released, did not support alpha transparency. This deficiency was rectified in Internet Explorer 7, bringing Internet Explorer in line with other web browsers in this regard.