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.
Even the implementation is also different like [Context menu's image](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L4558-L4595).
If you need to implement it in a variety of forms, make a sort of API using [`css var()`](https://developer.mozilla.org/en-US/docs/Web/CSS/var).
Problems that are not [`appearance: auto`](https://developer.mozilla.org/en-US/docs/Web/CSS/appearance#values) may require emulation like [Win7, 8's menu active color](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L146-L223).
[Bookmark menu](https://github.com/black7375/Firefox-UI-Fix/issues/136) is also similar example([code](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L4745-L4840)).
### Firefox Version
There may be changes that change by version of Firefox.
It's [more frequent than your thought](https://github.com/black7375/Firefox-UI-Fix/issues?q=is%3Aissue+label%3Aupstream+), and we need to respond to compatibility between [Nightly](https://www.mozilla.org/en-US/firefox/nightly/notes/) and [ESR](https://www.mozilla.org/en-US/firefox/organizations/notes/) versions.
This project is using SCSS to make a [reusable compatible mixins](../src/utils).
- Breaking change with `-moz-accent-color`/`-moz-accent-color-foreground` -> `AccentColor`/`AccentColorText` [#433](https://github.com/black7375/Firefox-UI-Fix/issues/433)
### `-moz-accent-color`
I don't know the exact reason, but it can be a problem if it is not applied to [CSS Variable](https://developer.mozilla.org/en-US/docs/Web/CSS/var). [#437](https://github.com/black7375/Firefox-UI-Fix/issues/437)
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).
Especially be careful when customizing XUL elements.
The following code will cause extension panels fail to open and trying to open them might even crash Firefox. ([@MrOtherGuy](https://www.reddit.com/r/FirefoxCSS/comments/u1mdld/comment/i4eg29n/?utm_source=share&utm_medium=web2x&context=3) reports)
```css
#nav-bar {
display: flex;
}
```
Info: `#nav-bar` is [`toolbar`](https://udn.realityripple.com/docs/Archive/Mozilla/XUL/toolbar).
Another `display: flex`'s side effect examples. [#368](https://github.com/black7375/Firefox-UI-Fix/issues/368) [#372](https://github.com/black7375/Firefox-UI-Fix/issues/372)
## Internals
### 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. [#11](https://github.com/black7375/Firefox-UI-Fix/issues/11)
### DOM structure cannot be modified
It is possible with [JS(Autoconfig)](./Preference.md#auto-config), 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.
For icons, [`list-style-image`](https://developer.mozilla.org/en-US/docs/Web/CSS/list-style-image) and [`background-image`](https://developer.mozilla.org/en-US/docs/Web/CSS/background-image) are available.
It is recommended to use `list-style-image` when a layout is provided for the image.
`background-image` may require many calculations to fit the layout.
You can use [`-moz-box-ordinal-group`](https://udn.realityripple.com/docs/Web/CSS/box-ordinal-group)([`-moz-box`](https://udn.realityripple.com/docs/Web/CSS/Mozilla_Extensions#display)) and [`order`](https://developer.mozilla.org/en-US/docs/Web/CSS/order)(at [flex](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_Flexible_Box_Layout/Basic_Concepts_of_Flexbox)) to change the order of the layout.
In experience, the detailed actions of `-moz-box` and `flex` are different, so tests are required.
- [Sync Menu Item's Image Order](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L4234-L4238)
- [Tabs On Bottom](https://github.com/black7375/Firefox-UI-Fix/blob/c453ef43a699759f55800a5d266a89ac11321b2b/src/tabbar/_tabs_on_bottom.scss#L4-L32)
Finally, it's time to respond to various states.
By default, [attribute selectors](https://developer.mozilla.org/en-US/docs/Web/CSS/Attribute_selectors) is used, and sometimes [hard coding](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L873-L893) according to [selector specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity) may be required.
If there is no state attributes, you can bypass by using [both transitions and keyframes](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userChrome.css#L2704-L2730) for animation.
- [Audio, Video Player UI](https://github.com/black7375/Firefox-UI-Fix/blob/36e9c94844fee2417662251cbd50c2b874d5b576/userContent.css#L5-L47)
- [Video Player Twoline UI](https://github.com/black7375/Firefox-UI-Fix/blob/cbf14cd55a9edada7ab2f5f1b626608fb9fe38a2/src/contents/_video_player.scss#L68-L171): It is pretty difficult when the condition becomes complicated.
Another limitation of shadow dom in user style is that you cannot use shadow dom related selectors like [`:host`](https://developer.mozilla.org/en-US/docs/Web/CSS/:host_function) and [`::part`](https://developer.mozilla.org/en-US/docs/Web/CSS/::part).
- [Using ::part() selector in userchrome.css?](https://www.reddit.com/r/FirefoxCSS/comments/d2sukj/using_part_selector_in_userchromecss/)
- [Can't change some shadow-dom properties](https://www.reddit.com/r/FirefoxCSS/comments/rebn3s/cant_change_some_shadowdom_properties/)
- [Bug 1575507 - Shadow parts should work in user-origin stylesheets.](https://bugzilla.mozilla.org/show_bug.cgi?id=1575507)
### XUL
Sometimes firefox can use [XUL](https://en.wikipedia.org/wiki/XUL) that have been written and binded with C++ for performance like a treeview of bookmarks.
XUL's [box model](https://udn.realityripple.com/docs/Archive/Mozilla/XUL/Tutorial/The_Box_Model) and [DOM](https://udn.realityripple.com/docs/Archive/Mozilla/XUL/Tutorial/Document_Object_Model) are different from HTML.
There ar few appropriate documents, so we have to read the source code and work. (Ex. [1](https://github.com/mozilla/gecko-dev/blob/master/layout/style/nsCSSAnonBoxList.h), [2](https://github.com/mozilla/gecko-dev/blob/master/layout/xul/tree/nsITreeView.idl))
Like [`<toolbar align="end"></toolbar>`](https://udn.realityripple.com/docs/Archive/Mozilla/XUL/Attribute/align), [`attributes`](https://udn.realityripple.com/docs/Archive/Mozilla/XUL/Attribute) is set and CSS of same property may not be appplied. (Ex. [`box-align: start`](https://udn.realityripple.com/docs/Web/CSS/box-align))
### Supports
[`@supports`](https://developer.mozilla.org/en-US/docs/Web/CSS/@supports) change in CSS is not detected in real time. (Caching after checking only once)
So a restart is required, and if the mozilla need real time changes, are using `@media` to handle it.
If project only use pure CSS, we cannot add `@media rules`.
- [Bug 1267890 - Support detecting bool preferences in chrome stylesheets](https://bugzilla.mozilla.org/show_bug.cgi?id=1267890)
- [Bug 1698132 - Improve caching behaviour of -moz-bool-pref](https://bugzilla.mozilla.org/show_bug.cgi?id=1698132)
This restriction is also related to the [Doc: Preference.md](./Preference.md#using-with-user-custom-css).
### Namespace
In older codes, the following [namespace](https://developer.mozilla.org/en-US/docs/Web/CSS/@namespace) is commonly seen:
However, it is [only applicable to XUL](https://www.userchrome.org/adding-style-recipes-userchrome-css.html#namespaces), so it is recommended to use with `prefix`.
There are a few caveats when you [`@import`](https://developer.mozilla.org/en-US/docs/Web/CSS/@import) the CSS.
It's because of specification definition, not Firefox design, but to prevent some mistakes.
`@import` rule is not a [nested statement](https://developer.mozilla.org/en-US/docs/Web/CSS/Syntax#nested_statements). Therefore, it cannot be used inside [conditional group at-rules](https://developer.mozilla.org/en-US/docs/Web/CSS/At-rule#conditional_group_rules).
Any [`@namespace`](https://developer.mozilla.org/en-US/docs/Web/CSS/@namespace) rules must follow all `@charset` and @import rules, and precede all other at-rules and style declarations in a style sheet.
