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 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!(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 and ::after can indirectly add CSS elements.
Shadow DOM
- Firefox actively uses shadow dom internally
- To modify, it is often a roundabout approach or impossible to inherit
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.