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+# CSS
+
+This page is for information about CSS used by DevTools. Wondering about the Dev Edition theme? See this page for more information about the [Developer Edition theme](https://wiki.mozilla.org/DevTools/Developer_Edition_Theme).
+
+## Basics
+
+The CSS code is in `devtools/client/themes`.
+
+Here are some basic tips that can optimize reviews if you are changing CSS:
+
+* Avoid `!important` but if you have to use it, make sure it's obvious why you're using it (maybe with a comment).
+* Avoid magic numbers, prefer automatic sizing.
+* Avoid platforms specific styles, put everything in the `shared` directory.
+* Avoid preprocessor variables, use CSS variables instead.
+* Avoid setting styles in JavaScript. It's generally better to set a class and then specify the styles in CSS
+* `classList` is generally better than `className`. There's less chance of over-writing an existing class.
+
+### Boilerplate
+
+Make sure each file starts with the standard copyright header (see [License Boilerplate](https://www.mozilla.org/MPL/headers/)).
+
+### Testing
+
+CSS changes should generally be similar across platforms since they used a shared implementation, but there can still be differences worth checking out. Check major changes on Windows, OS X and Ubuntu.
+
+## Formatting
+
+We use 2-spaces indentation for the CSS.
+
+In general the formatting looks like this:
+
+```css
+selector,
+alternate-selector {
+  property: value;
+  other-property: other-value;
+}
+```
+<!--TODO: add examples for long shorthand properties, and multi-valued properties (background, font-family, ...)-->
+Also:
+
+* Omit units on 0 values.
+ * Example: Use `margin: 0;`, not `margin: 0px;`.
+* Add a space after each comma, **except** within color functions.
+ * Example: `linear-gradient(to bottom, black 1px, rgba(255,255,255,0.2) 1px)`.
+* Always add a space before ` !important`.
+* Assume `="true"` in attribute selectors.
+ * Example: Use `option[checked]`, not `option[checked="true"]`.
+* Use longhand versions of properties so it's clear what you're changing.
+ * Example: Use `border-color: red`, not `border: red;`.
+
+Naming standards for class names:
+
+* `lower-case-with-dashes` is the most common.
+* But `camelCase` is also used sometimes. Try to follow the style of existing or related code.
+
+## Light and Dark theme support
+
+DevTools supports 2 different themes: the dark theme and the light theme. In order to support them, there are 2 class names available (`theme-dark` and `theme-light`).
+
+* Use [pre-defined CSS variables](https://searchfox.org/mozilla-central/source/devtools/client/themes/variables.css) instead of hardcoding colors when possible.
+* If you need to support themes and the pre-defined variables don't fit, define a variable with your custom colors at the beginning of the CSS file. This avoids selector duplication in the code.
+
+Example:
+
+```css
+.theme-light {
+  --some-variable-name: <color-for-light-theme>;
+}
+.theme-dark {
+  --some-variable-name: <color-for-dark-theme>;
+}
+#myElement {
+  background-color: var(--some-variable-name);
+}
+```
+
+## HDPI support
+
+It's recommended to use SVG since it keeps the CSS clean when supporting multiple resolutions. However, if only 1x and 2x PNG assets are available, you can use this `@media` query to target higher density displays (HDPI): `@media (min-resolution: 1.1dppx)`. <!--TODO an example would be good here-->
+
+## Performance
+
+* Use an iframe where possible so your rules are scoped to the smallest possible set of nodes.<!--TODO: is this still true? and also refine exactly when it is appropriate to use an iframe. Examples might help-->
+* If your CSS is used in `browser.xhtml`, you need to take special care with performance:
+ * Descendent selectors should be avoided.
+ * If possible, find ways to use **only** id selectors, class selectors and selector groups.
+
+## Localization
+
+### Text Direction
+* For margins, padding and borders, use `inline-start`/`inline-end` rather than `left`/`right`.
+ * Example: Use `margin-inline-start: 3px;` not `margin-left: 3px`.
+* For RTL-aware positioning (left/right), use `inset-inline-start/end`.
+* When there is no special RTL-aware property (eg. `float: left|right`) available, use the pseudo `:-moz-locale-dir(ltr|rtl)` (for XUL files) or `:dir(ltr|rtl)` (for HTML files).
+* Remember that while a tab content's scrollbar still shows on the right in RTL, an overflow scrollbar will show on the left.
+* Write `padding: 0 3px 4px;` instead of `padding: 0 3px 4px 3px;`. This makes it more obvious that the padding is symmetrical (so RTL won't be an issue).
+
+### RTL support for html modules
+
+By default, new HTML modules support only left-to-right (LTR) and do not reuse the current direction of the browser.
+
+To enable right-to-left (RTL) support in a module, set the `[dir]` attribute on the document element of the module:
+* Example: `<html xmlns="http://www.w3.org/1999/xhtml" dir="">`.
+
+The appropriate value for the `dir` attribute will then be set when the toolbox loads this module.
+
+### Testing
+
+The recommended workflow to test RTL on DevTools is to use the [Force RTL extension](https://addons.mozilla.org/en-US/firefox/addon/force-rtl/). After changing the direction using Force RTL, you should restart DevTools to make sure all modules apply the new direction. A future version of Force RTL will be able to update dynamically all DevTools documents.<!--TODO: update when the fate of this addon/webextension is known-->
+
+## Toggles
+
+Sometimes you have a style that you want to turn on and off. For example a tree twisty (a expand-collapse arrow), a tab background, etc.
+
+The Mozilla way is to perform the toggle using an attribute rather than a class:
+
+```css
+.tree-node {
+  background-image: url(right-arrow.svg);
+}
+.tree-node[open] {
+  background-image: url(down-arrow.svg);
+}
+```
+
+## Tips
+
+* Use `:empty` to match a node that doesn't have children.
+* Usually, if `margin` or `padding` has 4 values, something is wrong. If the left and right values are asymmetrical, you're supposed to use `-start` and `-end`. If the values are symmetrical, use only 3 values (see localization section).