Chrome Version: 71.0.3578.20 
OS: All

What steps will reproduce the problem?
(1) Go To https://roettsch.es/devtools_stylesheet_update.html
(2) Open DevTools
(3) Observe .fontsize_update class in Element Styles
(4) Move the slider

What is the expected result?
Font Size changes, rendering updated, DevTools should show updated font-size value for .fontsize_update class.

What happens instead?
.fontsize_update stays at font-size: 20px, even though text size has changed.