Default is defined. 

https://folio.googleplex.com/chrome-ux-specs-and-sources/Chrome%20browser%20(MD)/Secondary%20UI%20Previews%20and%20specs%20(exports)#%2FSPEC-secondary-UI-01a-buttons-light-theme.png%3Fz=width

That default seems kind of weird to me; it looks half-disabled. The fact it matches hover is confusing because in many places we reuse the same effect for hover and focus. Granted, we use a blue ring for focus here, but as a user I might reasonably look at a default button and think it was the focused button.

I'm still holding out hope that we'd not have a default that was separate from call to action, i.e., all dialogs would get call to action styling on their default buttons. Has that already been rejected?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7c4e09087ab718cc98ca7917fa7e27eca359b0b1

commit 7c4e09087ab718cc98ca7917fa7e27eca359b0b1
Author: estade <estade@chromium.org>
Date: Sun Aug 21 01:05:34 2016

Adjust MdTextButton border calculation.

Buttons should normally be 28dp tall. Other scenarios with bigger than
usual text require adjustment to that value.

BUG= 635151 

Review-Url: https://codereview.chromium.org/2256403002
Cr-Commit-Position: refs/heads/master@{#413357}

[modify] https://crrev.com/7c4e09087ab718cc98ca7917fa7e27eca359b0b1/ui/views/controls/button/md_text_button.cc
[modify] https://crrev.com/7c4e09087ab718cc98ca7917fa7e27eca359b0b1/ui/views/controls/button/md_text_button.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e4187f4aa33438f9dca1b96638ff5b651632a12e

commit e4187f4aa33438f9dca1b96638ff5b651632a12e
Author: estade <estade@chromium.org>
Date: Wed Sep 07 05:11:30 2016

Harmony - update button focus rings.

Effective changes: always 2dp wide, more opaque.

BUG= 635151 

Review-Url: https://codereview.chromium.org/2311423002
Cr-Commit-Position: refs/heads/master@{#416847}

[modify] https://crrev.com/e4187f4aa33438f9dca1b96638ff5b651632a12e/chrome/browser/ui/views/download/download_item_view_md.cc
[modify] https://crrev.com/e4187f4aa33438f9dca1b96638ff5b651632a12e/ui/views/controls/button/md_text_button.cc
[modify] https://crrev.com/e4187f4aa33438f9dca1b96638ff5b651632a12e/ui/views/controls/button/md_text_button.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e84ef9b0ce92cf74aaad872583e1e757dfb95ad3

commit e84ef9b0ce92cf74aaad872583e1e757dfb95ad3
Author: estade <estade@chromium.org>
Date: Wed Sep 07 23:57:05 2016

Sharpen MdTextButton border for fractional scale factors.

BUG= 635611 , 635151 

Review-Url: https://codereview.chromium.org/2322653002
Cr-Commit-Position: refs/heads/master@{#417122}

[modify] https://crrev.com/e84ef9b0ce92cf74aaad872583e1e757dfb95ad3/ui/views/controls/button/md_text_button.cc
[modify] https://crrev.com/e84ef9b0ce92cf74aaad872583e1e757dfb95ad3/ui/views/controls/focusable_border.cc
[modify] https://crrev.com/e84ef9b0ce92cf74aaad872583e1e757dfb95ad3/ui/views/painter.cc

More or less done. Any further tweaks can be tracked in more specific bugs.