def allowance(browser):
    """
    Our image diffs below give some false alarms of diffs... The values
    below are rmsdiff values for 100%-same ImageChops histograms (i.e.,
    ImageChops' bounding box method showed no difference), and for
    trivial rendering differences, e.g., a pixel within a button gradient
    differing, or rendered text differing very slightly, we add a slight
    allowance.
    """
    margin = 1.001
    margins = {'default': 572.4334022399462 * margin, 'chrome': 
        572.4334022399462 * margin, 'firefox': 957.864291014 * margin}
    try:
        return margins[browser]
    except KeyError:
        return margins['default']
