Chrome Version       : 59.0.3071.115
OS Version: 10.0
URLs (if applicable) :
Other browsers tested: Only Tested in Chrome 59.0.3071.115

What steps will reproduce the problem?
1. create a dialog element
2. add a css custom property to root ( :root { --custom-bg: rgba(0, 0, 0, 0.8); }
3. use that custom prop for the dialog's backdrop: ( dialog::backdrop { background: var(--custom-bg);

What is the expected result?
The root custom prop would be used to set the dialog's background as seen on https://demo.agektmr.com/dialog/#background_color

What happens instead of that?
no props are passed from any top level selectors

Please provide any additional information below. Attach a screenshot if
possible.

UserAgentString: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.115 Safari/537.36