Re: [csswg-drafts] [css-backgrounds] Add a 'hairline' border-width value (#3720)

I do think the supporting for fractional order `px` value, means 0.1~0.9px (may be even more precise than this), is more useful than we finally only have a new `hairline` key word value for `border-width` property.

As on mobile web, Android app can use `0.3dp` or `0.5dp` , and iOS app can use `0.3pt` or `0.5pt` to express the hairline line. For CSS, `px` just play the same role as `dp` on Android and `pt` on iOS.

I wish, one day, we can use `0.3px` or `0.5px` to match the exact border width with the mobile apps, so the users can see the same thick or thin lines on the their Apps.

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