How to style :root based on Custom Element Attribute

Question

Update: See: How to style :root without !important using proper specificity

Is it not possible?

This Style rule is totally ignored

Answer 1

You forgot the parenthesis to :host():

elem.attachShadow({mode: 'open'})
    .innerHTML = `
    <style>
      :host([player="X"]) { 
        display: inline-block;
        background: red;
        padding 1em; 
      }
    </style>
    Hello World`

<game-toes id=elem player="X"></game-toes>