https://github.com/w3c/csswg-drafts/issues/1680
https://drafts.csswg.org/css-easing/#step-timing-functions
<step-timing-function> = step-start | step-end | steps(<integer>[, <step-position>]?)
<step-position> = jump-start | jump-end | jump-none | jump-both | start | end

The jump-* names are new from the CSS Working Group's Berlin F2F in April 2018.