TODO:

• A simple bento.css stylesheet deployed to CDN.
• A bento.js script that installs stylesheet and provides some utilities.
• Reduce stylesheet declaration duplications.
• Include boilerplate CSS in per-element stylesheets
• Decide whether amp-inline-gallery and amp-inline-gallery-pagination and such should continue separate.
• Include CSS for inlines: amp-date-display, amp-timeago, amp-date-countdown.
• Figure out if we need to cancel stylesheet install when <link rel=stylesheet> is available, and how.