Aurelia Shadow DOM slots (Blog). GitHub Gist: instantly share code, notes, and snippets.
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Luckily, we don’t have to. Shadow DOM supports slot elements, that are automatically filled by the content from light DOM. Let’s see how slots work on a simple example. Here, shadow DOM provides two slots, filled from light DOM.

index.js

// Custom Template/Slot without Shadow DOM

// template refers to <template></template>

// context refers to stuff inside <your-custom-element> <span slot='bork'>🍭</span> </your-custom-element>

// overall flow of operation translates to YOUR-HTML = TEMPLATE(CONTEXT)

// a map of all <slot name='bork'> elements, where key is the name attribute, and value the <slot name='bork'> node;

consttemplate=newMap(Array.from(this.querySelectorAll('slot').values()).map(i=>[i.name,i]));

// an array of [[name, element]] where name is the slot attribute of html element <span slot='bork'>🍭</span>

constcontext=Array.from(this.querySelectorAll(':scope > *[slot]').values()).map(i=>[i.slot,i]);

console.log(context)

// traverse context, the list of elements with slot='*' property

// name is taken from context (see above)

// element is the thing we want to replace <slot> with

for(const[slotName,element]ofcontext){

// slot is the <slot name='bork'> referenced by <span slot='bork'>🍭</span> inside your custom element

constslot=template.get(slotName);

// if template had the slot bork, replace the entire <slot name='bork'>*</slot> with <span slot='bork'>🍭</span>

if(slot)slot.parentElement.replaceChild(element,slot);// Syntax: replacedNode = parentNode.replaceChild(newChild, oldChild);

// remove the remove slot='bork' from <span slot='bork'>🍭</span>

// NOTE: THIS IS NON SPEC, YOU SHOULD COMMENT THIS OUT FOR FUTURE COMPAT

// element.removeAttribute('slot');

}

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Shadow DOM may include both <style> and <link href='…'> tags. In the latter case, stylesheets are HTTP-cached, so they are not redownloaded for multiple components that use same template.

As a general rule, local styles work only inside the shadow tree, and document styles work outside of it. But there are few exceptions.

:host

The :host selector allows to select the shadow host (the element containing the shadow tree).

For instance, we’re making <custom-dialog> element that should be centered. For that we need to style the <custom-dialog> element itself.

That’s exactly what :host does:

Cascading

The shadow host (<custom-dialog> itself) resides in the light DOM, so it’s affected by document CSS rules.

If there’s a property styled both in :host locally, and in the document, then the document style takes precedence.

For instance, if in the document we had:

…Then the <custom-dialog> would be without padding.

It’s very convenient, as we can setup “default” component styles in its :host rule, and then easily override them in the document.

Poker toernooien nederland 2018. The exception is when a local property is labelled !important, for such properties, local styles take precedence.

:host(selector)

Same as :host, but applied only if the shadow host matches the selector.

For example, we’d like to center the <custom-dialog> only if it has centered attribute:

Now the additional centering styles are only applied to the first dialog: <custom-dialog centered>.

:host-context(selector)

Same as :host, but applied only if the shadow host or any of its ancestors in the outer document matches the selector.

E.g. :host-context(.dark-theme) matches only if there’s dark-theme class on <custom-dialog> on anywhere above it:

To summarize, we can use :host-family of selectors to style the main element of the component, depending on the context. These styles (unless !important) can be overridden by the document.

Styling slotted content

Now let’s consider the situation with slots.

Slotted elements come from light DOM, so they use document styles. Local styles do not affect slotted content.

Slots Without Shadow Domain

In the example below, slotted <span> is bold, as per document style, but does not take background from the local style:

The result is bold, but not red.

If we’d like to style slotted elements in our component, there are two choices.

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Web Components Slots Without Shadow Dom

First, we can style the <slot> itself and rely on CSS inheritance:

Here <p>John Smith</p> becomes bold, because CSS inheritance is in effect between the <slot> and its contents. But in CSS itself not all properties are inherited.

Another option is to use ::slotted(selector) pseudo-class. It matches elements based on two conditions:

That’s a slotted element, that comes from the light DOM. Slot name doesn’t matter. Just any slotted element, but only the element itself, not its children.
The element matches the selector.

In our example, ::slotted(div) selects exactly <div slot='username'>, but not its children:

Please note, ::slotted selector can’t