In the video Stef talks about disabling CSS in Chrome to see the impact it has on a website. However this is an older version of Chrome and I couldn't find a tool to do this in the current version (maybe that's my inexperience).
I did find the extension "disable-HTML" in the Chrome web store by Daniel Lucks (link below). It's actually an elegantly simple control panel for this extension with straight forward "on/off" buttons for not just CSS but also JavaScript, images, cookies and pop-ups-note pop-ups are defaulted to off when you install it.
https://chrome.google.com/webstore/detail/disable-html/lfhjgihpknekohffabeddfkmoiklonhm