::Slotted CSS Selector For Nested Children in Shadowdom Slot

::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>

<my-element>
  <div slot=Hello>Hello reflected (slotted) lightDOM DIV</div>
</my-element>

<my-element>
  <div slot=NoSlot>Hello not slotted lightDOM DIV</div>
</my-element>

<script>
  customElements.define('my-element', class extends HTMLElement {
    constructor() {
      super()
        .attachShadow({mode: 'open'})
        .innerHTML=`<slot name=Hello><b>I am shadowDOM SLOT content</b></slot>`;
    }
    connectedCallback() {
      console.log(this.shadowRoot.querySelector('slot').innerHTML)
    }
  });
</script>

Select all text:
<div id=selectable>
  <x-widget>CONTENT1</x-widget>
  <x-widget><div>CONTENT2</div></x-widget>
</div>
<style>
  body {
    --selectionBackground: green; --selectionColor: white;
    font-size: 2em;
  }
  #selectable ::selection {
    background: var(--selectionBackground); color: var(--selectionColor);
    font-weight: bold;
  }
</style>
<script>
  window.customElements.define('x-widget', class extends HTMLElement {
    constructor() {
      const template = document.createElement('template');
      template.innerHTML = `<div>TEMPLATE</div><div><slot></slot></div>`;
      const style = document.createElement('style');
      style.textContent = `
            ::selection { /* colors only template text, not slot content */
                background: var(--selectionBackground);
                color: var(--selectionColor);
            }
            ::slotted(*) {  /* selectors select HTMLElements! */
                color: red; /* CONTENT1 is TEXT, NOT an HTMLElement! */
            }`;
      super().attachShadow({mode: 'open'})
             .append(style, template.content.cloneNode(true));
    }});
</script>

var element = document.querySelector('.templateMe');
var shadow = element.attachShadow({mode: 'open'});
var template = document.querySelector('.myTemplate');
shadow.appendChild(template.content.cloneNode(true));

<template class="myTemplate">
  <style type="text/css">
    ::slotted([slot=slot1]) { /* slot1 every slotted element - YES */
      color: red;
    }
    
    slot[name=slot1] { /* slot1 itself - YES */
      text-decoration: underline;
    }
    
    slot[name=slot1] + .siblingA { /* slot1 siblingA (direct sibling) - YES */
      color: green;
    }
    
    slot[name=slot1]  ~ .siblingB { /* slot1 siblingB (any sibling) - YES */
      color: orange;
    }
    
    slot[name=slot2]::slotted(.selectMeA) { /* slot2 TOP-LEVEL CHILD (slotted) - YES */
      color: purple;
    }
    
    slot[name=slot2]::slotted(.selectMeB) { /* slot2 NOT TOP-LEVEL CHILD - NO */
      font-weight: bold;
    }
    
    slot[name=slot2]::slotted(.selectMeC[name=myName]) { /* slot2 TOP-LEVEL CHILD (slotted) - YES */
      color: khaki;
    }
    
    slot[name=slot2] + .siblingC { /* slot2 sibling - YES */
      color: blue;
    }
    
  </style>
  <div>
    <slot name="slot1"></slot>
    <div class="siblingA">Sibling A of Slot 1</div>
    <div class="siblingB">Sibling B of Slot 1</div>
  </div>
  <hr/>
  <div>
    <slot name="slot2"></slot>
    <div class="siblingC">Sibling C of Slot 2</div>
  </div>
</template>

<div class='templateMe'>
  <span slot="slot1">Im in Solt 1</span>
  <span slot="slot2" class="selectMeA">
    Im in Solt 2, im selectable.
    <div class='selectMeB'>
      NOT selectable (because no top level node of slotted)!
    </div>
  </span>
  <span slot="slot2" class="selectMeC" name="myName">Im in Solt 2 too and selectable!</span>
</div>

    0px <vertical-layout id="Level1" childmargin="15px">
    15px  <div>child1-1</div>
    15px  <div>child1-2</div>
    15px  <div>child1-3</div>
    15px  <vertical-layout id="Level2" childmargin="10px">
    0px    <div>child2-1</div>
    10px   <div>child2-2</div>
    10px   <vertical-layout id="Level3" childmargin="5px">
    5px      <div>child3-1</div>
    5px      <div>child3-2</div>
    5px      <div>child3-3</div>
           </vertical-layout>
    10px    <div>child2-3</div>
         </vertical-layout>
    15px <div>child1-4</div>
    15px <div>child1-5</div>
        </vertical-layout>

    connectedCallback() {
      let margin = this.getAttribute("childmargin");
      setTimeout(() => {
        let children = [...this.querySelectorAll("*:not(:first-child)")];
        children.forEach(child=>child.style.setProperty("--childmargin", margin));
      });
    }

[...this.children].forEach((child,idx)=>{
  if(idx) ....
};

  vertical-layout > *:not(:first-child)  {
    margin-top: var(--childmargin);
  }

customElements.define("vertical-layout", class extends HTMLElement {
  constructor() {
    super()
     .attachShadow({mode:"open"})
     .innerHTML = "<style>:host{display:flex;flex-direction:column}</style><slot></slot>";
  }
  connectedCallback() {
    let margin = this.getAttribute("childmargin");
    setTimeout(() => {
        let children = [...this.querySelectorAll("*:not(:first-child)")];
        children.forEach(child=>child.style.setProperty("--childmargin", margin));
      });
  }
});

<vertical-layout id="Level1" childmargin="15px">
  <div>child1-1</div>
  <div>child1-2</div>
  <div>child1-3</div>
  <vertical-layout id="Level2" childmargin="10px">
    <div>child2-1</div>
    <div>child2-2</div>
    <vertical-layout id="Level3" childmargin="5px">
      <div>child3-1</div>
      <div>child3-2</div>
      <div>child3-3</div>
    </vertical-layout>
    <div>child2-3</div>
  </vertical-layout>
  <div>child1-4</div>
  <div>child1-5</div>
</vertical-layout>

<style>
  body {
    font: 12px arial;
  }
  vertical-layout > *:not(:first-child)  {
    font-weight: bold;
    margin-top: var(--childmargin);
  }
  vertical-layout::before {
    content: "<vertical-layout " attr(id) " childmargin=" attr(childmargin);
  }
  vertical-layout > vertical-layout {
    background: lightblue;
    border-top: 4px dashed red;
  }
  vertical-layout > vertical-layout > vertical-layout {
    background: lightcoral;
  }
</style>

<script>
  customElements.define("vertical-layout", class extends HTMLElement {
    constructor() {
      super()
        .attachShadow({
          mode: "open"
        })
        .innerHTML =
        `<style>
         :host {
           display: flex;
           flex-direction: column;
           background:lightgreen;
           padding-left:20px;
           border:2px solid red;
         }
         ::slotted(*){margin-left:20px}
         :host([childmargin]) ::slotted(:not(:first-child)) {
           color:blue;
         }
       </style>
       <slot>
       <slot></slot>
       </slot>`;
    }
    connectedCallback() {
      let margin = this.getAttribute("childmargin");
      setTimeout(() => {
        let children = [...this.querySelectorAll("*:not(:first-child)")];
        children.map(child=>{
          child.style.setProperty("--childmargin", margin);
          child.append(` margin-top: ${margin}`);
        })
      });
    }
  });

</script>

const MATCH_END = /(<\/[a-zA-Z][a-zA-Z0-9_-]*>)$/;

/**
 * Reconstruct the innerHTML of a shadow element
 */
export function reconstruct_shadow_slot_innerHTML(el: HTMLElement): string {
    return reconstruct_shadow_slotted(el).join('').replace(/\s+/, ' ');
}

export function reconstruct_shadow_slotted(el: Element): string[] {
    const child_nodes = el.shadowRoot ? el.shadowRoot.childNodes : el.childNodes;
    return reconstruct_from_nodeList(child_nodes);
}

function reconstruct_from_nodeList(child_nodes: NodeList|Node[]): string[] {
    const new_values = [];
    for (const child_node of Array.from(child_nodes)) {
        if (!(child_node instanceof Element)) {
            if (child_node.nodeType === Node.TEXT_NODE) {
                // text nodes are typed as Text or CharacterData in TypeScript
                new_values.push((child_node as Text).data);
            } else if (child_node.nodeType === Node.COMMENT_NODE) {
                const new_data = (child_node as Text).data;
                new_values.push('<!--' + new_data + '-->');
            }
            continue;
        } else if (child_node.tagName === 'SLOT') {
            const slot = child_node as HTMLSlotElement;
            new_values.push(...reconstruct_from_nodeList(slot.assignedNodes()));
            continue;
        } else if (child_node.shadowRoot) {
            new_values.push(...reconstruct_shadow_slotted(child_node));
            continue;
        }
        let start_tag: string = '';
        let end_tag: string = '';

        // see @syduki's answer to my Q at
        // https://stackoverflow.com/questions/66618519/getting-the-full-html-for-an-element-excluding-innerhtml
        // for why cloning the Node is much faster than doing innerHTML;
        const clone = child_node.cloneNode() as Element;  // shallow clone
        const tag_only = clone.outerHTML;
        const match = MATCH_END.exec(tag_only);
        if (match === null) {  // empty tag, like <input>
            start_tag = tag_only;
        } else {
            end_tag = match[1];
            start_tag = tag_only.replace(end_tag, '');
        }
        new_values.push(start_tag);
        const inner_values: string[] = reconstruct_from_nodeList(child_node.childNodes);
        new_values.push(...inner_values);
        new_values.push(end_tag);
    }
    return new_values;
}

const MATCH_END = /(<\/[a-zA-Z][a-zA-Z0-9_-]*>)$/;


/**
 * Reconstruct the innerHTML of a shadow element
 */
function reconstruct_shadow_slot_innerHTML(el) {
    return reconstruct_shadow_slotted(el).join('').replace(/\s+/, ' ');
}

function reconstruct_shadow_slotted(el) {
    const child_nodes = el.shadowRoot ? el.shadowRoot.childNodes : el.childNodes;
    return reconstruct_from_nodeList(child_nodes);
}


function reconstruct_from_nodeList(child_nodes) {
    const new_values = [];
    for (const child_node of Array.from(child_nodes)) {
        if (!(child_node instanceof Element)) {
            if (child_node.nodeType === Node.TEXT_NODE) {
                new_values.push(child_node.data);
            } else if (child_node.nodeType === Node.COMMENT_NODE) {
                const new_data = child_node.data;
                new_values.push('<!--' + new_data + '-->');
            }
            continue;
        } else if (child_node.tagName === 'SLOT') {
            const slot = child_node;
            new_values.push(...reconstruct_from_nodeList(slot.assignedNodes()));
            continue;
        } else if (child_node.shadowRoot) {
            new_values.push(...reconstruct_shadow_slotted(child_node));
            continue;
        }
        let start_tag = '';
        let end_tag = '';

        const clone = child_node.cloneNode();
        // shallow clone
        const tag_only = clone.outerHTML;
        const match = MATCH_END.exec(tag_only);
        if (match === null) {  // empty tag, like <input>
            start_tag = tag_only;
        } else {
            end_tag = match[1];
            start_tag = tag_only.replace(end_tag, '');
        }
        new_values.push(start_tag);
        const inner_values = reconstruct_from_nodeList(child_node.childNodes);
        new_values.push(...inner_values);
        new_values.push(end_tag);
    }

    return new_values;
}

class HelloThere extends HTMLElement {
   constructor() {
      super();
      const shadow = this.attachShadow({mode: 'open'});
      shadow.innerHTML = '<span>Hello <slot></slot>!</span>';
   }
}

customElements.define('hello-there', HelloThere);

<hello-there><b>S</b>amantha</hello-there>
<div>Output: <input type="text" size="200" id="output"></input></div>
<script>
const ht = document.querySelector('hello-there');
const out = document.querySelector('#output');

</script>
<button onclick="out.value = ht.innerHTML">InnerHTML hello-there</button><br>
<button onclick="out.value = ht.outerHTML">OuterHTML hello-there</button><br>
<button onclick="out.value = ht.shadowRoot.innerHTML">InnerHTML hello-there shadow</button><br>
<button onclick="out.value = ht.shadowRoot.outerHTML">OuterHTML hello-there shadow (property does not exist)</button><br>
<button onclick="out.value = reconstruct_shadow_slot_innerHTML(ht)">Desired output</button>

  customElements.define('my-table', class extends HTMLElement {
    constructor() {
      let template = document.cloneNode(document.querySelector('#t1').content, true);
      super()
       .attachShadow({ mode: 'open' })  
       .append(template);
    }
  })

<template id="t1">
  <style>
    :host {
      display: flex;
      padding:1em;
    }
    ::slotted(*:first-child) {
      background: green;
    }
    ::slotted(*:last-child) {
      background: yellow;
      flex:1;
    }
    ::slotted(*:first-of-type) {
      border: 2px solid red;
    }
    ::slotted(*:last-of-type) {
      border: 2px dashed red;
    }
  </style>
  <slot name="column"></slot>
</template>

<my-table>
  <div slot="column">Alpha</div><div slot="column">Bravo</div><div slot="column">Charlie</div>
</my-table>
<my-table>
  <div slot="column">Delta</div><div slot="column">Echo</div>
</my-table>