CSS include support for image-set

[haidubogdan]

• adding support for CSS image-set. (Css Image Module Level 4 https://www.w3.org/TR/css-images-4/#funcdef-image-set)
• include 'x' unit for resolution property.

Closes #8343

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[matthiasblaesing]

Looks sane to me. Thank you!

Minimal nitpick for the future: the commit message should have a summary line then an empty line and then the text with further information, closes/signed-off by lines etc.