In Responsive Design Mode, Page Style applies to the browser UI, not to the website

[Kuba Orlik]

I was viewing a website in Responsive Design Mode (via devtools). I wanted to test how the site would look like without any styles attached, so I picked "View -> Page Styles -> No style". But the removal of styles was applied to the browser UI, not to the website itself:

[Kenji Okamoto]

This has been filed in bugzilla 1500923