New property to control the direction of slider controls

[mfreed7]

I'm extracting this question from the various discussions in several forums:

• [css-pseudo-4] Standardizing input[type="range"] styling #4410 (comment)
• Rendering <input type=range> vertically whatwg/html#4177 (comment)
• Clarify value direction for elements "progress" and "meter" with vertical writing mode whatwg/html#8413 (comment)
• [switch]: Should author defined content be allowed on tracks or thumb? openui/open-ui#959

For slider controls (<input type=range>, <progress>, <meter>, potentially <input type=checkbox switch>, etc.) there is a desire to be able to control:

1. The orientation of the slider: horizontal vs. vertical.
2. The polarity/direction of the slider: which way is toward "more".
3. The writing mode and direction of any text associated with the control, such as tooltips.

In current browsers, #1 is controlled by the CSS writing-mode property, and #2 is controlled by the CSS direction property. This does allow full control of orientation and direction for sliders, and at least recently (thanks @dizhang168), this behavior is implemented interoperably, despite a lack of standards for this behavior. However, because of #3 (see whatwg/html#4177 (comment) in particular), it might make sense to have a specific CSS property to control the orientation and direction of slider type controls. One tricky detail will be getting it to behave predictably in the face of existing writing-mode/direction behavior, and maybe even the non-standard orient=vertical.

[yisibl]

Placeholders.

I will add a detailed proposal here later.

[annevk]

Given that https://w3c.github.io/aria/#aria-orientation exists I wonder to what extent we should consider this to be a presentational aspect of the control. Maybe orient=vertical is the right approach?