https://drafts.csswg.org/css-fonts-4/#font-weight-prop describes animation type as discrete, without explicitly specifying the discrete steps, but probably refering to the 100,200,300,400,500,600,700,800,900 increments of CSS Fonts Level 3.
We should follow up on this in hub.com/w3c/csswg-drafts/labels/css-fonts-4
Comment 1 by drott@chromium.org
, Jul 5 2017