Support for creating custom properties as functions about stylex HOT 2 CLOSED

Unnecessary properties occur when an approach to the declared --color-slot-1 inside the svg is assumed

from stylex.

This is an intentional part of the design to ensure consistency and to enable some important features in the future. There's also a potential performance improvement we can make with the current design.

1. Consistency - It is a very complicated edge-case to explain, and TBH, I've forgotten some of the details, but TLDR; inline styles having the highest specificity can cause some problems.
2. Future features - We're planning on polyfilling future/experimental CSS features using a CSS post-processor. Some of these features are reliant on knowing all possible values a variable can take. Inline styles are not part of the CSS file and would cause issues. With the current design, the post-processing step can detect when a variable has a dynamic value and handle/error on that as appropriate.
3. Performance - Variable values are normally inherited, and this increases the performance cost of using variables. With the new @property feature, it possible to mark certain variables as non-inheriting. Using this enables us to speed up the usage of variables that are created for dynamic values.

from stylex.