Prio 1 (for next version)
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gallery
logout
login page

Prio 2 (for future versions)
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links in path line, to dirs
google pagespeed insights https://developers.google.com/speed/pagespeed/insights/?url=http%3A%2F%2Fwww.kneipenband.com%2Fwebbox%2F&tab=mobile
chromecast
player
sandclock during operations
i18n
forward/back button
list: more info if appropriate