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Doesn't really fit your 'outside any UI framework' requirement but there is http://reactdesktop.js.org/

Does a pretty good job at mimicking both Windows 10 and Mac OS. If there isn't a CSS framework someone else can link you to then you could either just use react (providing it's a project where that sort of decision doesn't affect anything) or attempt to take the CSS styles from this project.

While its not exactly Windows 10, MIcrosoft's design system is called "Fluent UI"[0] which includes what they use for the Web. It features a similar flat UI but isn't exactly a copy of Windows 10, it does it's own thing. The components are also unfortunately tied to React.

[0]https://developer.microsoft.com/en-us/fluentui#/

For what it's worth, it may be impossible to implement this using CSS features no more recent than 2005.

I’m imagining some horrible <table> layout now.