What Is the ::Content/::Slotted Pseudo-Element and How Does It Work

var div = document.querySelector('div');
var root = div.createShadowRoot();
var template = document.querySelector('template');

root.appendChild(template.content);

<template>
  <style>
    h3 { color: red; }
    content[select="h3"]::content > h3 { color: green; }
    ::content section p { text-decoration: underline; }
  </style>
  <h3>Shadow DOM</h3>
  <content select="h3"></content>
  <content select="section"></content>
</template>

<div>
  <h3>Light DOM</h3>
  <section>
    <div>I'm not underlined</div>
    <p>I'm underlined in Shadow DOM!</p>
  </section>
</div>

::slotted CSS selector for nested children in shadowDOM slot

styling ::slotted elements in shadowDOM

TL;DR

• ::slotted Specs: https://drafts.csswg.org/css-scoping/#slotted-pseudo

• slotted content remains in light DOM, is reflected to a <slot> in shadow DOM

• ::slotted(x) targets the lightDOM outer-Element (aka 'skin'), NOT the SLOT in shadowDOM

• ::slotted(x) takes basic selectors

• Inheritable styles trickle into shadowDOM
  
  https://lamplightdev.com/blog/2019/03/26/why-is-my-web-component-inheriting-styles/

• For the latest WHATWG discussion on SLOT and related topics, see

  • https://github.com/whatwg/html/issues/6051#issuecomment-816971072
    
    Participants: rniwa (Apple) , annvk (Mozilla), dominic (Google)
  • https://github.com/WICG/webcomponents/issues/934#issuecomment-906063140

background

Yes, ::slotted() not styling nested elements is expected behavior.

The term slotted is counterintuitive,

it implies element lightDOM is moved to shadowDOM

slotted lightDOM is NOT moved, it remains.. hidden.. in lightDOM

the content (IF slotted) is reflected to a <slot></slot>

Or from Google Developer Documentation

, .

' ; .

I use the term reflected instead of render because render implies you can access it in shadowDOM.

You can not, because slotted content isn't in shadowDOM... only reflected from lightDOM.

Why :slotted has limited functionality

More advanced shadowDOM styling was tried.

WebComponents version 0 (v0) had <content> and ::content; but it was removed from the spec:

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/content

The main takeway from the W3C standards discussions

(@hayatoito (Google team) here and here) is:

So in V1 we have :slotted: https://developer.mozilla.org/en-US/docs/Web/CSS/::slotted

Addition #1 : Performance if ::slotted allowed for complex selectors

From Mozilla developer Emilio:

source: https://github.com/w3c/webcomponents/issues/889

The performance issue is that it increments the amount of subtrees in
which every node needs to go look for rules that affect to them.

Right now the logic goes like: if you're slotted, traverse your slots
and collect rules in their shadow trees as needed. This is the code
This is nice because the complexity of styling the element
depends directly on the complexity of the shadow trees that you're
building, and it only affects slotted nodes.

If you want to allow combinators past slotted then every node would
need to look at its ancestor and prev-sibling chain and look at which
ones of them are slotted, then do that process for all their slots.
Then, on top, you also need to change the general selector-matching
code so that selectors that do not contain slotted selectors don't
match if you're not in the right shadow tree.

That's a cost that you pay for all elements, regardless of whether you
use Shadow DOM or ::slotted, and is probably just not going to fly.

So due to performance issues

:slotted( S ) got limited CSS selector functionality:

• ► it only takes simple selectors for S. --> Basically anything with a space won't work

• ► it only targets lightDOM 'skin'. --> In other words, only the first level

<my-element>
  <h1>Hello World</h1> 
  <p class=foo>
    <span>....</span>
  </p>
  <p class=bar>
    <span>....</span>
  </p>
</my-element>

• ::slotted(h1) and ::slotted(p) works

• ::slotted(.foo) works

• ::slotted(span) (or anything deeper) will not work (not a 'skin' element)

Note: ::slotted([Simple Selector]) confirms to Specificity rules,

but (being simple) does not add weight to lightDOM skin selectors, so never gets higher Specificity.

You might need !important in some (rare) use cases.

 <style>
  ::slotted(H1) {
    color: blue !important;
  }
 <style>

Styling slotted content

Also see: Applying more in depth selection to the :host CSS pseudo class

#1 - style lightDOM

The <span> is hidden in lightDOM, any changes made there will continue to reflect to its slotted representation.

That means you can apply any styling you want with CSS in the main DOM

(or a parent shadowDOM container if you wrapped <my-element> in one)

 <style>
  my-element span {
    .. any CSS you want
  }
 <style>

#2 - (workaround) move lightDOM to shadowDOM

If you move lightDOM to shadowDOM with: this.shadowRoot.append(...this.childNodes)

you can do all styling you want in a shadowDOM <style> tag.

Note: You can not use <slot></slot> and :slotted() anymore now.

<slot>s only works with content reflected from lightDOM.

For an example where an element wraps itself in an extra shadowDOM layer,

so no CSS bleeds out, and <slot>s can be used, see:

• https://jsfiddle.net/WebComponents/5w3o2q4t/?slotmeister

#3 - ::part (shadow Parts)

It is a different/powerful way of styling shadowDOM content:

Apple finally implemented shadowParts in Safari 13.1, March 2020

see:

• https://meowni.ca/posts/part-theme-explainer/

• https://css-tricks.com/styling-in-the-shadow-dom-with-css-shadow-parts/

• https://dev.to/webpadawan/css-shadow-parts-are-coming-mi5

• https://caniuse.com/mdn-html_global_attributes_exportparts

Note! ::part styles shadowDOM,

<slot></slot> content remains in lightDOM!

references

be aware: might contain v0 documentation!

• https://css-tricks.com/encapsulating-style-and-structure-with-shadow-dom/

• https://developers.google.com/web/fundamentals/web-components/shadowdom?hl=en#composition_slot

• https://polymer-library.polymer-project.org/2.0/docs/devguide/style-shadow-dom#style-your-elements

• https://github.com/w3c/webcomponents/issues/331

• https://github.com/w3c/webcomponents/issues/745

• https://developer.mozilla.org/en-US/docs/Web/API/HTMLSlotElement/slotchange_event

• ::part() - https://developer.mozilla.org/en-US/docs/Web/CSS/::part

Example: Using slots as a router

Change the slot-name on buttonclick and reflect content from lightDOM:

<template id=MY-ELEMENT>
  <style>
    ::slotted([slot="Awesome"]){
      background:lightgreen
    }
  </style>
  <slot><!-- all unslotted content goes here --></slot>
  <slot id=answer name=unanswered></slot>
</template>
<style>/* style all IMGs in lightDOM */
  img { max-height: 165px;border:3px dashed green }
  img:hover{ border-color:red }
</style>
<my-element><!-- content below is: lightDOM! -->
  SLOTs are: <button>Cool</button> <button>Awesome</button> <button>Great</button>
  <span slot=unanswered>?</span>
  <div  slot=Cool>   <img src="https://i.imgur.com/VUOujQT.jpg"></div>
  <span slot=Awesome><b>SUPER!</b></span>
  <div  slot=Awesome><img src="https://i.imgur.com/y95Jq5x.jpg"></div>
  <div  slot=Great>  <img src="https://i.imgur.com/gUFZNQH.jpg"></div>
</my-element>
<script>
  customElements.define('my-element', class extends HTMLElement {
    connectedCallback() {
      this.attachShadow({mode:'open'})
          .append(document.getElementById(this.nodeName).content.cloneNode(true));
      this.onclick = (evt) => {
           const label = evt.composedPath()[0].innerText; // Cool,Awesome,Great
           this.shadowRoot.getElementById("answer").name = label;
      }
    }
  });
</script>

function convertSelectorsForShadowDOM(selectors)
{
  var result = [];
  var prefix = "::content ";

    // If using shadow DOM, we have to add the ::content pseudo-element
    // before each selector, in order to match elements within the
    // insertion point.
    if (shadow)
      selectors = convertSelectorsForShadowDOM(selectors);

:nth-child(3), :nth-child(4), :nth-child(eleventy!!1!) {properties}

static get styles(): CSSResultArray {
    return [ 
      css`
        :host {
          /* styles */
        }

        ::slotted(span) {
          /* styles */
        }
      `,
      css`
        :host([data-type="primary"]) button {
          /* styles */
        }
      `
      ];
  }