I could have done this through a CSSLoader to allow all CSS unit quirks but
I wasn't planning to start passing around document and element references
everywhere, so instead just did it manually by accepting numbers/floats in
addition to pixel and percent.
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
targetFrame is modified during the intersection computation loop, so
it's not the viewport you want if there are scrollframes around.
This bug triggers when IntersectionObservers are used on frames that
wrap.
Follow-up for #249.
