dialog ::backdrop default background should adapt to `color-scheme: dark`

[danieltroger]

What is the issue with the HTML Standard?

I initially filed this as an issue to Chromium, however this seems to be the right place.

Please refer to the Chromium issue where I have described it in detail: https://issues.chromium.org/issues/369756811.

Problem Description (summary)

The default user agent styling for ::backdrop is

dialog:-internal-dialog-in-top-layer::backdrop {
    position: fixed;
    inset: 0px;
    background: rgba(0, 0, 0, 0.1);
}

regardless of what color-scheme is set to. According to MDN, color-scheme should affect

The default colors of other browser-provided UI

Therefore, the default background of ::backdrop should be set to rgba(0,0,0,0.4) or something else more noticeable, when color-scheme: dark; is set.

Click to see screenshots

Look at the console to see the current background value

current implementation, not visible enough☝