Consequently, I considered requiring JavaScript for a webpage to load properly to be bad practice. Even if I assume JavaScript is everywhere, I have yet to hear of how one would make things dependent on it accessible to the blind. I know that people will make webpages that require it regardless of whether the blind can view them or not, but I still feel that is bad practice.
The Polymer team's entire element catalog is accessible with ARIA integration done with assistance from Chrome's dedicated accessibility engineers. We even provide reusable abstractions that make writing your own accessible elements easier, like iron-a11y-announcer and iron-a11y-keys.
Accessibility was a big topic at our last Polymer Summit: https://www.youtube.com/watch?v=o6yLWihykVA