To take from javascript, this is a mix of underscore (utility) and, uhh, babel(?) for essentially polyfilling certain runtime features slated for future (hypothetical) approval.
The text is so faint on this web page I hardly want to get through it...
Reader view to the rescue
It's a sad commentary that this is now the de facto correct answer.
Optics => default to _Designer view_. Superflat everything; no affordances.