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CSS Shadow Parts

CSS Shadow Parts allow developers to style CSS properties on an element inside of a shadow tree. This is extremely useful in customizing Ionic Framework Shadow DOM components.

Why Shadow Parts?

Ionic Framework is a distributed set of Web Components. Web Components follow the Shadow DOM specification in order to encapsulate styles and markup.

Ionic Framework components are not all Shadow DOM components. If the component is a Shadow DOM component, there will be a badge in the top right of its component documentation. An example of a Shadow DOM component is the button component.

Shadow DOM is useful for preventing styles from leaking out of components and unintentionally applying to other elements. For example, we assign a .button class to our ion-button component. Without Shadow DOM encapsulation, if a user were to set the class .button on one of their own elements, it would inherit the Ionic Framework button styles. Since ion-button is a Shadow component, this is not a problem.

However, due to this encapsulation, styles aren’t able to bleed into inner elements of Shadow components either. This means that if a Shadow component renders elements inside of its shadow tree, the inner elements cannot be targeted directly with CSS. Using the ion-select component as an example, it renders the following markup:

<ion-select>
  #shadow-root
    <div class=”select-text select-placeholder”></div>
    <div class=”select-icon”></div>
</ion-select>
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The placeholder text and icon elements are inside of the #shadow-root, which means the following CSS will NOT work to style the placeholder:

/* Does NOT work */
ion-select .select-placeholder {
  color: blue;
}
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So how do we solve this? CSS Shadow Parts!

Shadow Parts Explained

Shadow parts allow developers to style inside a shadow tree, from outside of that shadow tree. In order to do so, the part must be exposed and then it can be styled by using ::part.

Exposing a part

When creating a Shadow DOM component, a part can be added to an element inside of a shadow tree by assigning a part attribute on the element. This is added to the component in Ionic Framework and requires no action from an end user.

Continuing to use the ion-select component as an example, the markup is updated to look like the following:

<ion-select>
  #shadow-root
    <div part=”placeholder” class=”select-text select-placeholder”></div>
    <div part=”icon” class=”select-icon”></div>
</ion-select>
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The above shows two parts: placeholder and icon. See the select documentation for all of its parts.

With these parts exposed, the element can now be styled directly using ::part.

How ::part works

The ::part() pseudo-element allows developers to select elements inside of a shadow tree that have been exposed via a part attribute.

Since we know that ion-select exposes a placeholder part for styling the text when there is no value selected, we can customize it in the following way:

ion-select::part(placeholder) {
  color: blue;
  opacity: 1;
}
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Styling using ::part allows any CSS property that is accepted by that element to be changed.

In addition to being able to target the part, pseudo-elements can be styled without them being explicitly exposed:

ion-select::part(placeholder)::first-letter {
  font-size: 22px;
  font-weight: 500;
}
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Parts work with most psuedo-classes, as well:

ion-item::part(native):hover {
  color: green;
}
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There are some known limitations with vendor prefixed pseudo-elements and structural pseudo-classes.

Ionic Framework Parts

All exposed parts for an Ionic Framework component can be found under the CSS Shadow Parts heading on its API page. To view all components and their API pages, see the Component documentation.

In order to have parts a component must meet the following criteria:

  • It is a Shadow DOM component. If it is a Scoped or Light DOM component, the child elements can be targeted directly. If a component is Scoped or Shadow, it will be listed by its name on its component documentation page.
  • It contains children elements. For example, ion-card-header is a Shadow component, but all styles are applied to the host element. Since it has no child elements, there’s no need for parts.
  • The children elements are not structural. In certain components, including ion-title, the child element is a structural element used to position the inner elements. We do not recommend customizing structural elements as this can have unexpected results.

We welcome recommendations for additional parts. Please create a new GitHub issue with as much information as possible when requesting a part.

Known Limitations

Browser Support

CSS Shadow Parts are supported in the recent versions of all of the major browsers. However, some of the older versions do not support shadow parts. Verify the browser support meets the requirements before implementing parts in an app. If browser support for older versions is required, we recommend continuing to use CSS Variables for styling.

Vendor Prefixed Pseudo-Elements

Vendor prefixed pseudo-elements are not supported at this time. An example of this would be any of the ::-webkit-scrollbar pseudo-elements:

/* Does NOT work */
my-component::part(scroll)::-webkit-scrollbar {
  background: green;
}
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See this issue on GitHub for more information.

Structural Pseudo-Classes

Most pseudo-classes are supported with parts, however, structural pseudo-classes are not. An example of structural pseudo-classes that do not work is below.

/* Does NOT work */
my-component::part(container):first-child {
  background: green;
}

/* Does NOT work */
my-component::part(container):last-child {
  background: green;
}
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Chaining Parts

The ::part() pseudo-element can not match additional ::part()s.

For example, my-component::part(button)::part(label) does not match anything. This is because doing so would expose more structural information than is intended.

If the <my-component>’s internal button uses something like part="label => button-label" to forward the button’s internal parts up into the panel’s own part element map, then a selector like my-component::part(button-label) would select just the one button’s label, ignoring any other labels.

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