All text/background combinations have a default contrast ratio that passes the minimum threshold of 4.5:1
1.4.3 Contrast (Minimum): The visual presentation of text and images of text has a contrast ratio of at least 4.5:1, except for the following: (Level AA)
Color values are derived from testing each of the provided schemes in Windows Vista, the resulting color values are indicated in the text boxes.
Tested by adding:opacity: 0.54 to the style declaration for input in the Firefox res/forms.css.
Note: colors have been set on the field text rather than the placeholder text for demonstration purposes.