props vs attrs

controllers.application\.js

import Controller from '@ember/controller';

export default class ApplicationController extends Controller {}

customElements.define('test-element', class extends HTMLElement {
  set known(value) {
    this.innerText +=
      `el.known = ${JSON.stringify(value)}`;
  } 
  
  setAttribute(name, value) {
    this.innerText +=
      `el.setAttribute("${name}", ${JSON.stringify(value)})`;
  }
});

styles.app\.css

body > :first-child {
  margin: 12px 16px;
  font-family: monospace;
  font-size: 12pt;
  white-space: pre;
  line-height: 1.5;
}

test-element {
  border: 1px solid #ddd;
  padding: 2px;
  border-radius: 4px;
}

templates.application\.hbs

known="hi"
<test-element known="hi"></test-element>

known=\{{"hi"}} 
<test-element known={{"hi"}}></test-element>

known="\{{"hi"}}"
<test-element known="{{"hi"}}"></test-element>

known="\{{123}}"
<test-element known="{{123}}"></test-element>

known=\{{123}}
<test-element known={{123}}></test-element>

unknown="hi"
<test-element unknown="hi"></test-element>

unknown=\{{"hi"}}
<test-element unknown={{"hi"}}></test-element>

unknown="\{{"hi"}}"
<test-element unknown="{{"hi"}}"></test-element>

unknown="\{{123}}"
<test-element unknown="{{123}}"></test-element>

unknown=\{{123}}
<test-element unknown={{123}}></test-element>

twiddle\.json

{
  "version": "0.17.1",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}