What are :horizontal and :vertical pseudo selectors for?

Question

I was writing some CSS in Visual Studio, and while writing a pseudo selector the code tooltip suggested :horizontal, so I noticed that there's a pseudo :horizontal and :vertical.

Visual Studio 2013 - 12.0.21005.1

What is this selector for, and when do I use it?

It does not appear on MDN Pseudo-Classes Docs standards selectors...

Answer 1

Is for custom scrollbar in webkit:

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:horizontal – The horizontal pseudo-class applies to any scrollbar pieces that have a horizontal orientation.

:vertical – The vertical pseudo-class applies to any scrollbar pieces that have a vertical orientation.

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:decrement – The decrement pseudo-class applies to buttons and track pieces. It indicates whether or not the button or track piece will decrement the view’s position when used (e.g., up on a vertical scrollbar, left on a horizontal scrollbar).

:increment – The increment pseudo-class applies to buttons and track pieces. It indicates whether or not a button or track piece will increment the view’s position when used (e.g., down on a vertical scrollbar, right on a horizontal scrollbar).

:start – The start pseudo-class applies to buttons and track pieces. It indicates whether the object is placed before the thumb.

:end – The end pseudo-class applies to buttons and track pieces. It indicates whether the object is placed after the thumb.

:double-button – The double-button pseudo-class applies to buttons and track pieces. It is used to detect whether a button is part of a pair of buttons that are together at the same end of a scrollbar. For track pieces it indicates whether the track piece abuts a pair of buttons.

:single-button – The single-button pseudo-class applies to buttons and track pieces. It is used to detect whether a button is by itself at the end of a scrollbar. For track pieces it indicates whether the track piece abuts a singleton button.

:no-button – Applies to track pieces and indicates whether or not the track piece runs to the edge of the scrollbar, i.e., there is no button at that end of the track.

:corner-present – Applies to all scrollbar pieces and indicates whether or not a scrollbar corner is present.

:window-inactive – Applies to all scrollbar pieces and indicates whether or not the window containing the scrollbar is currently active. (In recent nightlies, this pseudo-class now applies to ::selection as well. We plan to extend it to work with any content and to propose it as a new standard pseudo-class.)