I just randomly loaded a page which has no background colour explicitly set and… it got a transparent background. (It's a local page without any colour settings at all, intended to use the browser defaults.)
This bug is completely obscured if the element has a background colour explicitly set (by whatever means), but it can also be partly obscured:
<html>
<head>
<title>Test page</title>
</head>
<body>
<div style="background: yellow; color: red">This text has an explicitly-set background colour</div>
<div>This text gets the default background colour</div>
</body>
</html>
The default background colour should not be transparent!
Nothing extracted yet.
Steam client build: 1683926267
Devuan 4, Xfwm4 4.16.1.
Opening a separate Steam browser window then opening a new tab in that or changing the address to ”about:blank” causes the content area to become transparent. (The expected content is the default background colour filling the content area.)
It is properly transparent, rather than merely not rendered to: content below is rendered correctly. However, the window retains its normal shape, since interaction is with it and not with the content below.