Document the utility API's new css-var boolean

Co-Authored-By: Gaël Poupard <ffoodd@users.noreply.github.com>

4 files changed, 151 insertions, 41 deletions

diff --git a/site/content/docs/5.0/customize/css-variables.md b/site/content/docs/5.0/customize/css-variables.md
index 498bc85032..079f9ad23f 100644
--- a/site/content/docs/5.0/customize/css-variables.md
+++ b/site/content/docs/5.0/customize/css-variables.md
@@ -6,7 +6,7 @@ group: customize
 toc: true
 ---
 
-Bootstrap includes many [CSS custom properties (variables)](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) in its compiled CSS for real-time customization without the need to recomple Sass. These provide easy access to commonly used values like our theme colors, breakpoints, and primary font stacks when working in your browser's inspector, a code sandbox, or general prototyping.
+Bootstrap includes many [CSS custom properties (variables)](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties) in its compiled CSS for real-time customization without the need to recompile Sass. These provide easy access to commonly used values like our theme colors, breakpoints, and primary font stacks when working in your browser's inspector, a code sandbox, or general prototyping.
 
 **All our custom properties are prefixed with `bs-`** to avoid conflicts with third party CSS.
 
@@ -51,4 +51,4 @@ a {
 
 ## Grid breakpoints
 
-While we include our grid breakpoints as CSS variables (except for `xs`), be aware that **CSS variables do not work in media queries**. This is by design in the CSS spec for variables, but may change in coming years with support for `env()` variables. Check out [this Stack Overflow answer](https://stackoverflow.com/a/47212942) for some helpful links. In the mean team, you can use these variables in other CSS situations, as well as in your JavaScript.
+While we include our grid breakpoints as CSS variables (except for `xs`), be aware that **CSS variables do not work in media queries**. This is by design in the CSS spec for variables, but may change in coming years with support for `env()` variables. Check out [this Stack Overflow answer](https://stackoverflow.com/a/47212942) for some helpful links. In the mean time, you can use these variables in other CSS situations, as well as in your JavaScript.
diff --git a/site/content/docs/5.0/utilities/api.md b/site/content/docs/5.0/utilities/api.md
index 8ffc76caa3..e39d32fae7 100644
--- a/site/content/docs/5.0/utilities/api.md
+++ b/site/content/docs/5.0/utilities/api.md
@@ -12,16 +12,18 @@ Bootstrap utilities are generated with our utility API and can be used to modify
 The `$utilities` map contains all our utilities and is later merged with your custom `$utilities` map, if present. The utility map contains a keyed list of utility groups which accept the following options:
 
 {{< bs-table "table text-start" >}}
-| Option | Type | Description |
-| --- | --- | --- |
-| `property` | **Required** | Name of the property, this can be a string or an array of strings (e.g., horizontal paddings or margins). |
-| `values` | **Required** | List of values, or a map if you don't want the class name to be the same as the value. If `null` is used as map key, it isn't compiled. |
-| `class` | Optional | Variable for the class name if you don't want it to be the same as the property. In case you don't provide the `class` key and `property` key is an array of strings, the class name will be the first element of the `property` array. |
-| `state` | Optional | List of pseudo-class variants like `:hover` or `:focus` to generate for the utility. No default value. |
-| `responsive` | Optional | Boolean indicating if responsive classes need to be generated. `false` by default. |
-| `rfs` | Optional | Boolean to enable fluid rescaling. Have a look at the [RFS]({{< docsref "/getting-started/rfs" >}}) page to find out how this works. `false` by default. |
-| `print` | Optional | Boolean indicating if print classes need to be generated. `false` by default. |
-| `rtl` | Optional | Boolean indicating if utility should be kept in RTL. `true` by default. |
+| Option | Type | Default&nbsp;value | Description |
+| --- | --- | --- | --- |
+| [`property`](#property) | **Required** | – | Name of the property, this can be a string or an array of strings (e.g., horizontal paddings or margins). |
+| [`values`](#values) | **Required** | – | List of values, or a map if you don't want the class name to be the same as the value. If `null` is used as map key, it isn't compiled. |
+| [`class`](#class) | Optional | null | Name of the generated class. If not provided and `property` is an array of strings, `class` will default to the first element of the `property` array. |
+| [`css-var`](#css-variable-utilities) | Optional | `false` | Boolean to generate CSS variables instead of CSS rules. |
+| [`local-vars`](#local-css-variables) | Optional | null | Map of local CSS variables to generate in addition to the CSS rules. |
+| [`state`](#states) | Optional | null | List of pseudo-class variants (e.g., `:hover` or `:focus`) to generate. |
+| [`responsive`](#responsive) | Optional | `false` | Boolean indicating if responsive classes should be generated. |
+| `rfs` | Optional | `false` | Boolean to enable [fluid rescaling with RFS]({{< docsref "/getting-started/rfs" >}}). |
+| [`print`](#print) | Optional | `false` | Boolean indicating if print classes need to be generated. |
+| `rtl` | Optional | `true` | Boolean indicating if utility should be kept in RTL. |
 {{< /bs-table >}}
 
 ## API explained
@@ -40,7 +42,7 @@ $utilities: (
       100: 1,
     )
   )
- );
+);
 ```
 
 Which outputs the following:
@@ -53,9 +55,58 @@ Which outputs the following:
 .opacity-100 { opacity: 1; }
 ```
 
-### Custom class prefix
+### Property
+
+The required `property` key must be set for any utility, and it must contain a valid CSS property. This property is used in the generated utility's ruleset. When the `class` key is omitted, it also serves as the default class name. Consider the `text-decoration` utility:
+
+```scss
+$utilities: (
+  "text-decoration": (
+    property: text-decoration,
+    values: none underline line-through
+  )
+);
+```
+
+Output:
+
+```css
+.text-decoration-none { text-decoration: none !important; }
+.text-decoration-underline { text-decoration: underline !important; }
+.text-decoration-line-through { text-decoration: line-through !important; }
+```
+
+### Values
+
+Use the `values` key to specify which values for the specified `property` should be used in the generated class names and rules. Can be a list or map (set in the utilities or in a Sass variable).
+
+As a list, like with [`text-decoration` utilities]({{< docsref "/utilities/text#text-decoration" >}}):
+
+```scss
+values: none underline line-through
+```
+
+As a map, like with [`opacity` utilities]({{< docsref "/utilities/opacity" >}}):
+
+```scss
+values: (
+  0: 0,
+  25: .25,
+  50: .5,
+  75: .75,
+  100: 1,
+)
+```
+
+As a Sass variable that sets the list or map, as in our [`position` utilities]({{< docsref "/utilities/position" >}}):
+
+```scss
+values: $position-values
+```
+
+### Class
 
-Use the `class` option to change the class prefix used in the compiled CSS:
+Use the `class` option to change the class prefix used in the compiled CSS. For example, to change from `.opacity-*` to `.o-*`:
 
 ```scss
 $utilities: (
@@ -70,17 +121,76 @@ $utilities: (
       100: 1,
     )
   )
- );
+);
+```
+
+Output:
+
+```css
+.o-0 { opacity: 0 !important; }
+.o-25 { opacity: .25 !important; }
+.o-50 { opacity: .5 !important; }
+.o-75 { opacity: .75 !important; }
+.o-100 { opacity: 1 !important; }
+```
+
+### CSS variable utilities
+
+Set the `css-var` boolean option to `true` and the API will generate local CSS variables for the given selector instead of the usual `property: value` rules. Consider our `.text-opacity-*` utilities:
+
+```scss
+$utilities: (
+  "text-opacity": (
+    css-var: true,
+    class: text-opacity,
+    values: (
+      25: .25,
+      50: .5,
+      75: .75,
+      100: 1
+    )
+  ),
+);
+```
+
+Output:
+
+```css
+.text-opacity-25 { --bs-text-opacity: .25; }
+.text-opacity-50 { --bs-text-opacity: .5; }
+.text-opacity-75 { --bs-text-opacity: .75; }
+.text-opacity-100 { --bs-text-opacity: 1; }
+```
+
+### Local CSS variables
+
+Use the `local-vars` option to specify a Sass map that will generate local CSS variables within the utility class's ruleset. Please note that it may require additional work to consume those local CSS variables in the generated CSS rules. For example, consider our `.bg-*` utilities:
+
+```scss
+$utilities: (
+  "background-color": (
+    property: background-color,
+    class: bg,
+    local-vars: (
+      "bg-opacity": 1
+    ),
+    values: map-merge(
+      $utilities-bg-colors,
+      (
+        "transparent": transparent
+      )
+    )
+  )
+);
 ```
 
 Output:
 
 ```css
-.o-0 { opacity: 0; }
-.o-25 { opacity: .25; }
-.o-50 { opacity: .5; }
-.o-75 { opacity: .75; }
-.o-100 { opacity: 1; }
+.bg-primary {
+  --bs-bg-opacity: 1;
+  background-color: rgba(var(--bs-primary-rgb), var(--bs-bg-opacity)) !important;
+}
 ```
 
 ### States
@@ -116,7 +226,7 @@ Output:
 .opacity-100-hover:hover { opacity: 1 !important; }
 ```
 
-### Responsive utilities
+### Responsive
 
 Add the `responsive` boolean to generate responsive utilities (e.g., `.opacity-md-25`) across [all breakpoints]({{< docsref "/layout/breakpoints" >}}).
 
@@ -133,7 +243,7 @@ $utilities: (
       100: 1,
     )
   )
- );
+);
 ```
 
 Output:
@@ -186,21 +296,7 @@ Output:
 }
 ```
 
-### Changing utilities
-
-Override existing utilities by using the same key. For example, if you want additional responsive overflow utility classes, you can do this:
-
-```scss
-$utilities: (
-  "overflow": (
-    responsive: true,
-    property: overflow,
-    values: visible hidden scroll auto,
-  ),
-);
-```
-
-### Print utilities
+### Print
 
 Enabling the `print` option will **also** generate utility classes for print, which are only applied within the `@media print { ... }` media query.
 
@@ -217,7 +313,7 @@ $utilities: (
       100: 1,
     )
   )
- );
+);
 ```
 
 Output:
@@ -246,6 +342,20 @@ All utilities generated by the API include `!important` to ensure they override
 
 Now that you're familiar with how the utilities API works, learn how to add your own custom classes and modify our default utilities.
 
+### Override utilities
+
+Override existing utilities by using the same key. For example, if you want additional responsive overflow utility classes, you can do this:
+
+```scss
+$utilities: (
+  "overflow": (
+    responsive: true,
+    property: overflow,
+    values: visible hidden scroll auto,
+  ),
+);
+```
+
 ### Add utilities
 
 New utilities can be added to the default `$utilities` map with a `map-merge`. Make sure our required Sass files and `_utilities.scss` are imported first, then use the `map-merge` to add your additional utilities. For example, here's how to add a responsive `cursor` utility with three values.
diff --git a/site/content/docs/5.0/utilities/background.md b/site/content/docs/5.0/utilities/background.md
index d5edbdf3b3..61c27365b1 100644
--- a/site/content/docs/5.0/utilities/background.md
+++ b/site/content/docs/5.0/utilities/background.md
@@ -52,7 +52,7 @@ Consider our default `.bg-success` utility.
 }
 ```
 
-We use an RGB version of our `--bs-succes` (with the value of `25, 135, 84`) CSS variable and attached a second CSS variable, `--bs-bg-opacity`, for the alpha transparency (with no default value, but a fallback of `1`). That means anytime you use `.bg-success` now, your computed `color` value is `rgba(25, 135, 84, 1)`.
+We use an RGB version of our `--bs-success` (with the value of `25, 135, 84`) CSS variable and attached a second CSS variable, `--bs-bg-opacity`, for the alpha transparency (with a default value `1` thanks to a local CSS variable). That means anytime you use `.bg-success` now, your computed `color` value is `rgba(25, 135, 84, 1)`. The local CSS variable inside each `.bg-*` class avoids inheritance issues so nested instances of the utilities don't automatically have a modified alpha transparency.
 
 ### Example
 
diff --git a/site/content/docs/5.0/utilities/colors.md b/site/content/docs/5.0/utilities/colors.md
index 9502c48777..60462070b4 100644
--- a/site/content/docs/5.0/utilities/colors.md
+++ b/site/content/docs/5.0/utilities/colors.md
@@ -48,7 +48,7 @@ Consider our default `.text-primary` utility.
 }
 ```
 
-We use an RGB version of our `--bs-primary` (with the value of `13, 110, 253`) CSS variable and attached a second CSS variable, `--bs-text-opacity`, for the alpha transparency (with no default value, but a fallback of `1`). That means anytime you use `.text-primary` now, your computed `color` value is `rgba(13, 110, 253, 1)`.
+We use an RGB version of our `--bs-primary` (with the value of `13, 110, 253`) CSS variable and attached a second CSS variable, `--bs-text-opacity`, for the alpha transparency (with a default value `1` thanks to a local CSS variable). That means anytime you use `.text-primary` now, your computed `color` value is `rgba(13, 110, 253, 1)`. The local CSS variable inside each `.text-*` class avoids inheritance issues so nested instances of the utilities don't automatically have a modified alpha transparency.
 
 ### Example