UserAgent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:61.0) Gecko/20100101 Firefox/61.0

Steps to reproduce the problem:
None. There is no way to disable certain CSS features to test fallbacks and progressive enhancement.

What is the expected behavior?
There should be the option to disable CSS features (properties and values). Ideally there is a panel/tab where all the CSS features are listed (maybe grouped like Flexbox, Grid, Transforms...) and for every property and value there is a checkbox to disable the feature.

An example: A user disables "Grid" - in this case the browser will ignore display: grid | inline-grid | subgrid; as well as all grid-* properties as long as the checkbox is checked. At the same time all CSS defined inside @supports (display: grid) will be ignored and CSS.supports("display", "grid"); would also return false.

Another example: A user disables the value "rgba()" in this case the browser will ignore all rgba() values as long as the checkbox is checked. At the same time all CSS defined inside @supports (color: rgba(255, 0, 0, 0.3)) will be ignored and CSS.supports("color", "rgba(255, 0, 0, 0.3)"); would also return false.

What went wrong?
None, this is a feature request.

Did this work before? No 

Chrome version:   Channel: n/a
OS Version: 10.0
Flash Version: