Doc: CONTRIBUTING - Restrictions

CONTRIBUTING.md

@@ -12,6 +12,7 @@
   - [Project Structure](#project-structure)
     - [Icon files](#icon-files)
     - [Meta Info files](#meta-info-files)
+  - [Restrictions](#restrictions)
 - [Rules](#rules)
   - [Version](#version)
   - [Branch](#branch)
@@ -167,6 +168,27 @@ Path=<Full PATH>
 - Release(<git tag>): force latest tag update
 - Git<git hash>: latest commit update
 
+### Restrictions
+
+- Cross Platform
+  - Different compatibility issues occur in Win7, Win8, Win10, KDE, Gnome, Mac, etc.
+  - Consider compatibility as much as possible, but use [dedicated media queries](https://github.com/mozilla/gecko-dev/blob/d6188c9ce02efeea309e7177fc14c9eb2f09db37/servo/components/style/gecko/media_features.rs#L906-L930) in special cases
+- CSS Loading Order
+  - User CSS(`userChrome.css`, `userContent.css`) is usually loaded first.
+  - In many cases, overriding should be prevented with [`important!`](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity#the_!important_exception)(Anti-pattern in general web), and side effects should also be considered.
+- DOM structure cannot be modified
+  - It is possible with JS, but there are security and configuration issues, so we should make the most of CSS.
+  - [`::before`](https://developer.mozilla.org/en-US/docs/Web/CSS/::before) and [`::after`](https://developer.mozilla.org/en-US/docs/Web/CSS/::after) can indirectly add CSS elements.
+- [Shadow DOM](https://developer.mozilla.org/en-US/docs/Web/Web_Components/Using_shadow_DOM)
+  - Firefox actively uses shadow dom internally
+  - To modify, it is often a roundabout approach or impossible to inherit
+- [XUL](https://en.wikipedia.org/wiki/XUL)
+  - Sometimes written and bound in C++ for performance, like a treeview of bookmarks.
+  - The proper document does not exist, so we have to read the source code and work
+  - Available CSS features are also restricted.
+- Side Effects
+  - Only CSS modifications can cause bugs that are hard to think of in the general web, such as the [context menu not appearing](https://github.com/black7375/Firefox-UI-Fix/issues/114).  
+
 ## Rules
 
 ### Version