Add fallbacks for position: sticky

The WebKit engine (used by Apple’s Safari and GNOME Web aka Epiphany)
doesn’t support position: sticky without prefix yet. Also, add position:
fallback as another rudimentary fallback – it looks ugly (the row with
the buttons is no longer stretched out across the entire width), but is
still better than the alternative of not having the buttons on the
screen without scrolling at all.

Thanks to Sjoerd de Bruin for the hint!