Shadow DOM

<div class="v"><header>STYLE BOUNDARY <span id="outerLabel">outer: blue</span></header><div class="stage shadow"><div class="outer" id="outer">document style<template id="tpl"><div class="inner">shadow card</div></template><div id="host"></div></div></div><div class="muted">outer selector stops at shadow root</div></div>

.v{width:min(94vw,760px);height:min(88vh,320px);padding:clamp(9px,2.4vmin,18px);border:1px solid var(--line);border-radius:12px;background:var(--surface);font:500 clamp(12px,2.4vmin,16px)/1.3 var(--font-sans,system-ui,sans-serif);display:flex;flex-direction:column;gap:clamp(5px,1.4vmin,10px);overflow:hidden}.v header{display:flex;align-items:center;justify-content:space-between;gap:8px;color:var(--accent);font-weight:700}.v header span{color:var(--muted);font-weight:500}.v .stage{flex:1;min-height:0;position:relative}.v .muted{color:var(--muted)}.v .active{background:color-mix(in srgb,var(--accent) 18%,var(--surface));border-color:var(--accent)!important;color:var(--fg)}.v button{font:inherit;color:var(--fg);background:var(--bg);border:1px solid var(--line);border-radius:7px;padding:5px 8px;cursor:pointer}.shadow{display:grid;place-items:center}.outer{width:70%;min-height:75%;padding:9px;border:2px dashed var(--accent-2);border-radius:9px;color:var(--accent-2);display:flex;align-items:center;justify-content:center;flex-direction:column;gap:7px;transition:color .5s,border-color .5s}.outer.hot{color:var(--accent-3);border-color:var(--accent-3)}#host{width:65%;display:block}

const host=document.getElementById("host");host.attachShadow({mode:"open"}).innerHTML="<style>.inner{padding:8px;text-align:center;color:white;background:#0e9f6e;border-radius:7px;font:600 13px system-ui}</style><div class=inner>shadow card</div>";let hot=false;function tick(){hot=!hot;document.getElementById("outer").classList.toggle("hot",hot);document.getElementById("outerLabel").textContent=hot?"outer: amber":"outer: blue"}tick();setInterval(tick,1100)