CSS: How to target ::slotted siblings in Shadow DOM root?

Question

I know that the spec currently only allows compound selectors for ::slotted, i.e. ::slotted(my-first + my-second) is not allowed, but should something like this be working?

::slotted(x-first) + ::slotted(x-second) { /* css */ }

Is there any way to target slotted siblings (other than with global css)? And if not, where would I file such a request? Thanks.

Answer 1

Sure you can select siblings of slots / slotted.

The thing you can not do is select a element which has been slotted and is not a top-level node.

Select siblings:

slot[name=<slotname>] ~ <selector>

Select slotted top-level node

::slotted(<compound-selector>)

A compound-selector contains a tag/class/id/name etc. but must not have any combinators. Like <space> for example.

.myClass OK

<anyTag>[<anyAttribute>[=<anyValue>]] OK

.<myClass> > .<anotherClass> NO

Examples

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>

More here.

slotted elements (coming from light DOM), ::slotted(selector) allows to select slotted elements themselves, but not their children.