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

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(<simple selector>)

A simple 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 itself - YES */
      color: red;
    }
    
    slot[name=slot1] { /* slot1 itself (alternative) - 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>

Answer 2

DOM that is placed into a slot is supposed to be controlled by the CSS that owns that DOM and not by the Custom Element.

You are allowed very minor CSS control over the DOM that is placed into your Slot. But only the top level elements (And things that are auto inherited by child nodes.)

This was a conscious decision and will probably never change.