docs(core): add guidelines for naming CSS shadow parts

docs/README.md

@@ -23,6 +23,7 @@ It is based on <a href="https://www.webcomponents.org/introduction">Web Componen
 | [Contributing](./CONTRIBUTING.md)       | How to contribute including creating pull requests, commit message guidelines, and more. |
 | [Component Guide](./component-guide.md) | Guidelines for implementing component states, accessibility, and more.                   |
 | [Sass Guidelines](./sass-guidelines.md) | Outlines scenarios where Sass members and comments should be used.                       |
+| [CSS Shadow Parts Guidelines](./shadow-parts-guidelines.md) | Guidelines for CSS shadow parts in components.                       |
 
 ## Packages
 

docs/component-guide.md

@@ -17,6 +17,8 @@
   * [Example Components](#example-components-4)
   * [Component Structure](#component-structure-1)
 - [Converting Scoped to Shadow](#converting-scoped-to-shadow)
+- [Sass Variables](#sass-variables)
+- [CSS Shadow Parts](#css-shadow-parts)
 - [RTL](#rtl)
 - [Adding New Components with Native Input Support](#adding-new-components-with-native-input-support)
   * [Angular Integration](#angular-integration)
@@ -722,6 +724,14 @@ There will be some CSS issues when converting to shadow. Below are some of the d
 :host-context(ion-toolbar:not(.ion-color)):host(:not(.ion-color)) ::slotted(ion-segment-button) {
 ```
 
+## Sass Variables
+
+For guidelines on when to use Sass Variables, see the [Sass Guidelines](./sass-guidelines.md).
+
+## CSS Shadow Parts
+
+For guidelines on adding CSS shadow parts, see the [CSS Shadow Parts Guidelines](./shadow-parts-guidelines.md).
+
 ## RTL
 
 When you need to support both LTR and RTL modes, try to avoid using values such as `left` and `right`. For certain CSS properties, you can use the appropriate mixin to have this handled for you automatically.

docs/sass-guidelines.md

@@ -1,5 +1,6 @@
 # Sass Guidelines
 
+<sub><b>TABLE OF CONTENTS</b></sub>
 - [Definitions](#definitions)
 - [Scope](#scope)
 - [Historical Usage](#historical-usage)

docs/shadow-parts-guidelines.md

@@ -0,0 +1,277 @@
+# CSS Shadow Parts Guidelines
+
+<sub><b>TABLE OF CONTENTS</b></sub>
+- [Definitions](#definitions)
+- [Scope](#scope)
+- [General Guidelines](#general-guidelines)
+- [Standard Parts](#standard-parts)
+- [Specialized Parts](#specialized-parts)
+- [Documentation](#documentation)
+
+## Definitions
+
+**CSS Shadow Parts:** The CSS shadow parts module defines the [::part()](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Selectors/::part) pseudo-element that can be set on a [shadow host](https://developer.mozilla.org/en-US/docs/Glossary/Shadow_tree). Using this pseudo-element, you can enable shadow hosts to expose the selected element in the shadow tree to the outside page for styling purposes. [^1]
+
+## Scope
+
+Ionic Framework components that use Shadow DOM expose CSS Shadow Parts to enable custom styling by end users.
+
+This document establishes a standardized naming convention for CSS Shadow Parts in Ionic Framework components.
+
+## General Guidelines
+
+1. **Attempt to use standard parts first**: Use `native`, `wrapper`, `inner`, `container`, and `content` wherever they apply before inventing new names.
+2. **Use semantic, kebab-case names**: Choose descriptive names that communicate the role of the element (for example, `detail-icon`, `supporting-text`).
+3. **Reuse names for the same concept**: Use the same part name across components when the element serves the same role (for example, `backdrop`, `handle`, `label`).
+
+## Standard Parts
+
+**Name parts by what the element does, not where it appears.** Ask what role the element plays:
+
+| Name | Role |
+| --- | --- |
+| `native` | Is it a native HTML element that the user interacts with (e.g. `<button>`, `<a>`, `<input>`, `<textarea>`)? |
+| `wrapper` | Is it a native `<label>` element that wraps the whole form control? |
+| `inner` | Is it the inner layout wrapper around the main content? It may wrap only the default slot (e.g. `ion-list-header`), or a container plus slot(s) (e.g. `ion-item`, `ion-item-divider`, `ion-item-option`) when present. |
+| `container` | Does it wrap the main content itself (default slot or native control)? |
+| `content` | Is it the main user-content area of an overlay or primary content region? |
+
+The following examples shows the correct usage for the standard parts.
+
+### `native`
+
+**What it does:** The element the user directly interacts with - the native button, anchor, or form control (e.g. `<button>`, `<a>`, `<textarea>`, `<input>`).
+
+- **Use when**: The element receives click/focus/input from the user.
+- **Examples**: `ion-item`, `ion-button`, `ion-textarea`.
+
+**ion-item** - the interactive element is the `<button>`, `<a>`, or `<div>` (`TagType`):
+
+```tsx
+const TagType = clickable ? (href === undefined ? 'button' : 'a') : ('div' as any);
+
+return (
+  <Host>
+    <TagType class="item-native" part="native">
+      <slot name="start"></slot>
+      <div class="item-inner" part="inner">
+        <div class="input-wrapper" part="container">
+          <slot></slot>
+        </div>
+        <slot name="end"></slot>
+      </div>
+    </TagType>
+  </Host>
+);
+```
+
+**ion-textarea** - the interactive element is the native `<textarea>`:
+
+```tsx
+<div class="native-wrapper" part="container">
+  <textarea class="native-textarea" part="native">
+    {value}
+  </textarea>
+</div>
+```
+
+### `wrapper`
+
+**What it does:** The HTML `<label>` element that wraps the entire form control. Clicking anywhere on it focuses the control.
+
+- **Use when**: The element is the `<label>` that wraps the form control.
+- **Examples**: `ion-select`, `ion-textarea`, `ion-input`, `ion-checkbox`, `ion-toggle`, `ion-radio`, `ion-range`.
+
+**ion-select** - the `<label>` has `part="wrapper"`:
+
+```tsx
+<label class="select-wrapper" part="wrapper">
+  {this.renderLabelContainer()}
+  <div class="select-wrapper-inner" part="inner">
+    <slot name="start"></slot>
+    <div class="native-wrapper" part="container">...</div>
+    <slot name="end"></slot>
+  </div>
+</label>
+```
+
+**ion-textarea** - the `<label>` has `part="wrapper"`:
+
+```tsx
+<label class="textarea-wrapper" part="wrapper">
+  {this.renderLabelContainer()}
+  <div class="textarea-wrapper-inner" part="inner">
+    <slot name="start"></slot>
+    <div class="native-wrapper" part="container">...</div>
+    <slot name="end"></slot>
+  </div>
+</label>
+```
+
+### `inner`
+
+**What it does:** The inner layout wrapper around the main content. It may wrap only the default slot (e.g. `ion-list-header`), or it may wrap a container and the slot(s) (e.g. `start`, `end`) that sit alongside the main content. In `ion-item`, and `ion-item-divider`, the `start` slot is a sibling of this element. In `ion-select`, both `start` and `end` slots are inside this element.
+
+- **Use when**: The element is the inner layout wrapper (with or without a separate container and `start`/`end` slots).
+- **Examples**: `ion-list-header` (`.list-header-inner` wraps only the default slot), `ion-item` (`.item-inner`), `ion-item-divider` (`.item-divider-inner`), `ion-select` (`.select-wrapper-inner`).
+
+**ion-list-header** - `.list-header-inner` wraps only the default slot (no container, no `start`/`end` slots):
+
+```tsx
+<div class="list-header-inner" part="inner">
+  <slot></slot>
+</div>
+```
+
+**ion-item** - `.item-inner` wraps the container and `end` slot (`start` slot is a sibling):
+
+```tsx
+<slot name="start"></slot>
+<div class="item-inner" part="inner">
+  <div class="input-wrapper" part="container">
+    <slot></slot>
+  </div>
+  <slot name="end"></slot>
+</div>
+```
+
+**ion-item-divider** - `.item-divider-inner` wraps the container and `end` slot (`start` slot is a sibling):
+
+```tsx
+<slot name="start"></slot>
+<div class="item-divider-inner" part="inner">
+  <div class="item-divider-wrapper" part="container">
+    <slot></slot>
+  </div>
+  <slot name="end"></slot>
+</div>
+```
+
+**ion-select** - `.select-wrapper-inner` arranges `start` slot, container, `end` slot:
+
+```tsx
+<div class="select-wrapper-inner" part="inner">
+  <slot name="start"></slot>
+  <div class="native-wrapper" part="container"></div>
+  <slot name="end"></slot>
+</div>
+```
+
+### `container`
+
+**What it does:** Wraps the main content - either the default slot (for item-like components) or the native control and its immediate content (for form controls like select, textarea).
+
+- **Use when**: The element wraps the default slot, or wraps the native control (and any immediate content like listbox or slots inside it).
+- **Don’t use when**: The element is the main content area of an overlay (use `content` instead).
+- **Examples**: `ion-item` (`.input-wrapper` around default slot), `ion-item-divider` (`.item-divider-wrapper`), `ion-select` (`.native-wrapper` around select text + listbox), `ion-textarea` (`.native-wrapper` around `<textarea>`).
+
+From the examples above:
+
+**ion-item** - `.input-wrapper` wraps the default `<slot>`:
+
+```tsx
+<slot name="start"></slot>
+<div class="item-inner" part="inner">
+  <div class="input-wrapper" part="container">
+    <slot></slot>
+  </div>
+  <slot name="end"></slot>
+</div>
+```
+
+**ion-select** - `.native-wrapper` wraps the select text and listbox:
+
+```tsx
+<div class="select-wrapper-inner" part="inner">
+  <slot name="start"></slot>
+  <div class="native-wrapper" part="container">
+    {this.renderSelectText()}
+    {this.renderListbox()}
+  </div>
+  <slot name="end"></slot>
+</div>
+```
+
+**ion-textarea** - `.native-wrapper` wraps the `<textarea>`:
+
+```tsx
+<label class="textarea-wrapper" part="wrapper">
+  {this.renderLabelContainer()}
+  <div class="textarea-wrapper-inner" part="inner">
+    <slot name="start"></slot>
+    <div class="native-wrapper" part="container">
+      <textarea class="native-textarea" part="native">
+        {value}
+      </textarea>
+    </div>
+    <slot name="end"></slot>
+  </div>
+</label>
+```
+
+### `content`
+
+**What it does:** The main user-content area of an overlay or the primary content region (e.g. modal body, toolbar’s main slot).
+
+- **Use when**: The element is the primary content area where users see the main content (overlay body, or primary slot inside something like a toolbar).
+- **Examples**: `ion-modal`, `ion-popover`, `ion-accordion`, `ion-toolbar` (the div that wraps the default slot inside the toolbar container).
+
+**ion-modal** - `content` wraps the default `<slot>` which is the primary content:
+
+```tsx
+<div class="modal-content" part="content">
+  <slot></slot>
+</div>
+```
+
+**ion-toolbar** - `content` wraps the default `<slot>` which is the primary content:
+
+```tsx
+<div class="toolbar-container" part="container">
+  <slot name="start"></slot>
+  <slot name="secondary"></slot>
+  <div class="toolbar-content" part="content">
+    <slot></slot>
+  </div>
+  <slot name="primary"></slot>
+  <slot name="end"></slot>
+</div>
+```
+
+## Specialized Parts
+
+Components may also expose specialized parts for specific elements. The following parts are reused across multiple components:
+
+| Name | Description |
+| --- | --- |
+| `background` | Background elements (e.g., `ion-content`, `ion-toolbar`) |
+| `backdrop` | Backdrop elements. **Must only be used on `<ion-backdrop>` components.** (e.g., `ion-modal`, `ion-popover`, `ion-menu`) |
+| `label` | Label text elements - not the HTML `<label>` (see standard part `wrapper`) |
+| `supporting-text` | Supporting text elements |
+| `helper-text` | Helper text elements |
+| `error-text` | Error text elements |
+| `icon` | Icon elements. **Must only be used on `<ion-icon>` components.** Use specific names like `detail-icon`, `close-icon` when the icon serves a distinct purpose (e.g., `ion-item` uses `detail-icon`, `ion-fab-button` uses `close-icon`) |
+| `handle` | Handle elements (e.g., `ion-modal`, `ion-toggle`) |
+| `track` | Track elements (e.g., `ion-toggle`, `ion-progress-bar`) |
+| `mark` | Checkmark or indicator marks (e.g., `ion-checkbox`, `ion-radio`) |
+
+**When to create new specialized parts:**
+- Use standard parts (`native`, `wrapper`, `inner`, `container`, `content`) when they apply
+- Reuse existing specialized parts (listed above) when they match the element's role
+- Create component-specific specialized parts for elements that don't fit standard patterns or existing specialized parts
+- Use descriptive, semantic names (e.g., `header`, `text`, `arrow`, `scroll` for component-specific elements)
+
+## Documentation
+
+Shadow parts must be documented in the component's JSDoc comments using the `@part` tag. The following example demonstrates the proper documentation format:
+
+```tsx
+/**
+ * @part native - The native HTML button, anchor or div element that wraps all child elements.
+ * @part inner - The inner container element that wraps the item content.
+ * @part content - The wrapper element that contains the default slot.
+ * @part detail-icon - The chevron icon for the item. Only applies when `detail="true"`.
+ */
+```
+
+[^1]: MDN Documentation - CSS shadow parts, https://developer.mozilla.org/en-US/docs/Web/CSS/Guides/Shadow_parts