Created Procedural cosmetic filters (markdown)

Procedural-cosmetic-filters.md

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+The concept of procedural cosmetic filtering was introduced with uBlock Origin ("uBO") [version 1.8.0](https://github.com/gorhill/uBlock/releases/tag/1.8.0).
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+Normal, standard cosmetic filters are _declarative_, i.e. they are used as selector in a CSS rule, and completely handled by browsers through `style` tag elements.
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+_Procedural_ means javascript code is used to find DOM elements which must be hidden. A procedural cosmetic filter makes use of cosmetic filter _operator_, which will tell uBO how to find/filter DOM elements in order to find which DOM elements to target.
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+## Cosmetic filter operators
+
+### `subject:has(arg)`
+
+- Description: Select element `subject` if and only if evaluating `arg` in the context of `subject` returns one or more elements.
+- Subject `subject`: **must** be a plain CSS selector.
+- Argument `arg`: **must** be a plain CSS selector.
+- Examples:
+    - `strikeout.me##body > div:has(img[alt="AdBlock Alert"])`
+    - `yandex.ru##.serp-item:has(:scope > .organic > .organic__subtitle > .label_color_yellow)`
+
+The `:has(arg)` operator is actually a planned pseudo-class in CSS4, but as of writing no browser supports it. Instead of waiting for browser vendors to provide support, uBO provides support for `:has(arg)` as a procedural operator. By restricting `subject` and `arg` to be valid CSS selectors, this means uBO will be able to support the `:has(...)` operator declaratively once a browser supports it.
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