[css-backgrounds][css-logical-props] Consider defining logical values for <angle>

[jonathanKingston]

In the linear gradient syntax and transform rotate() function currently isn't able to use logical angles.

Such that to get a background to follow the direction of the text would require JavaScript.

If a developer were to use:

background: linear-gradient(logical 1deg, white, black);

The rotation of the linear gradient in rtl language might be -1deg.

[chharvey]

According to CSS Values and Units Module Level 3, an <angle> is always clockwise for positive values and counterclockwise for negative values. Furthermore, the value 0 refers to the bearing angle pointing "up" toward the "top" of the screen (equivalently true for 0deg, 0rad, 0turn, and 0grad).

I think the logical keyword should determine the direction of the zero-degree bearing angle: rather than "up", it should point to the "block-start" side of the screen. However, I disagree with changing the direction of the angle. I think positive angles should always be clockwise, regardless of text direction or writing mode.

[fantasai]

You might want to have a look at https://www.w3.org/TR/2012/WD-css4-images-20120911/#bidi-images and https://www.w3.org/TR/2017/WD-css-logical-1-20170518/#background-and-borders where we attempt to address this issue. Quite possible that we'd want to also have flow-relative angles, but I think there's a wide-ranging discussion that needs to happen here around the use cases and how best to address them all. The background-position property in Level 4, for example, allows fully-physical, fully-logical, and partially-logical coordinates in order to address a variety of concerns.