rotateyCSS | Python Fiddle

import math

time_per_img = 7.0
time_per_trans = 2.0
num_imgs = 4

anim_block = '''/* Animations */

{keyframes}
.side .usertext-body .md > ul:last-of-type a {{
 background:url(%%potw%%);
 opacity: 0;
 z-index: 0;
 -webkit-animation: fadeInOut1 {time}s linear infinite;
 animation: fadeInOut1 {time}s linear infinite;
}}

{imgblocks}'''
total_time = (time_per_img + time_per_trans) * num_imgs

time = int(total_time)

time_perc = 100 / total_time

on_perc = time_per_img * time_perc
trans_perc = time_per_trans * time_perc

def safeTime(t):
    if t < 0:
        t = 100 + t
    if t > 100:
        t = t - 100
    return t

def calcSlot(i):
    on = safeTime(i * (on_perc + trans_perc))
    startOff = safeTime(on + on_perc)

startOn = safeTime(on - trans_perc)
    startZOff = safeTime(startOn - 1)
    off = safeTime(startOff + trans_perc)
    endZOff = safeTime(off + 1)
    
    order = [math.floor(a) for a in (startZOff, startOn, on)] + \
        [math.ceil(a) for a in (startOff, off, endZOff)]
    commands = ['z-index:0', 'opacity:0;z-index:1', 'opacity:1',
                'opacity:1', 'opacity:0;z-index:1', 'z-index:0']
    d = dict((pos, commands[i]) for i, pos in enumerate(order))
    if 0 in d and 100 not in d:
        d[100] = d[0]
    elif 100 in d and 0 not in d:
        d[0] = d[100]
    return sorted(d.items(), key=lambda a: a[0])

keyframes = ''
imgblocks = ''
for i in range(num_imgs):
    percs = ' '.join(['{}%{{{}}}'.format(int(a), b) for a, b in calcSlot(i)])
    for prefix in ('', 'webkit', 'moz', 'o'):
        prefix = '-{}-'.format(prefix) if '' != prefix else ''
        keyframes += '''@{prefix}keyframes fadeInOut{animNum} {{
    {percs}
}}
'''.format(prefix=prefix, animNum=i + 1, percs=percs)
    if i > 0:
        imgblocks += '''.side .usertext-body .md > ul:last-of-type li:nth-of-type({i}) a{{
  animation-name: fadeInOut{i};
  -webkit-animation-name: fadeInOut{i};
  background:url(%%potw{i}%%);
}}
'''.format(i=i+1)

print anim_block.format(keyframes=keyframes, time=time, imgblocks=imgblocks)

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