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HTML

              
                <template id="template">
  <style>
    :host {
      --text-color: white;
      --background-color: red;
      --background-color-hover: darkred;
      --display: block;
      --width: 100%;
      --border-radius: 2px;
      --box-shadow: none;
    }

    .button {
      display: var(--display);
      width: var(--width);
      margin-bottom: 5px;
      padding: 1rem 1.5rem;
      color: var(--text-color);
      font-size: 1.2rem;
      background-color: var(--background-color);
      border-radius: var(--border-radius);
      box-shadow: var(--box-shadow);
      appearance: none;
      border: 0;
    }
    
    .button:hover {
      background-color: var(--background-color-hover);
    }
    
    @media only screen and (min-width: 40em) {
      .button {
        margin-bottom: 0;
      }
    }
  </style>
  <button class="button">
    Button
  </button>
</template>

<div class="faux-grid">
  <div class="stage">
    <some-elem></some-elem>
  </div>
  
  <div class="sidebar"></div>
</div>
              
            
!

CSS

              
                // Breakpoints Mixin
$breakpoints: (
  // small: var(--small)
  small: 0,
  medium: 40em,
  large: 64em
);

@mixin breakpoint($breakpoint) {
  @if map-has-key($breakpoints, $breakpoint) {
    @if $breakpoint == 'small' {
      @media only screen and (max-width: map-get($breakpoints, medium)) {
        @content;
      }
    }
    @else if $breakpoint == 'medium' {
      @media only screen and (min-width: map-get($breakpoints, medium)) {
        @content;
      }
    }
    @else if $breakpoint == 'large' {
      @media only screen and (min-width: map-get($breakpoints, large)) {
        @content;
      }
    }
  }
  
  @else {
    @warn 'Not a valid breakpoint: ' + $breakpoint;
  }
}

// Reset
* {
  box-sizing: border-box;
  *:before,
  *:after {
    box-sizing: inherit;
  }
}

body {
  margin: 1rem;
  text-align: center;
}

// Custom Element
some-elem {
  @include breakpoint(medium) {
    --display: inline-block;
    --width: auto;
    --background-color: green;
    --background-color-hover: darkgreen;
  }
    
  @include breakpoint(large) {
    --background-color: yellowgreen;
    --text-color: black;
  }
}

// For Demo Only
.faux-grid {
  display: flex;
  height: 100vh;
  border: 1px solid black;

  .stage {
    display: flex;
    justify-content: center;
    align-items: center;
    flex: 2;
  }
  
  .sidebar {
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-content: space-between;
    flex: 1;
    height: 100%;
    border-left: 1px solid black;
  }
}
              
            
!

JS

              
                // Web Component Declaration
class SomeElem extends HTMLElement {
  constructor() {
    super()

    const root = this.attachShadow({mode: 'open'})
    const temp = document.importNode(template.content, true)

    root.appendChild(temp)
  }
}

customElements.define('some-elem', SomeElem)

const customElement = document.querySelector('some-elem');
const sidebar = document.querySelector('.faux-grid .sidebar');
const elems = window.getComputedStyle(customElement);

// BUG IN CHROME - DOES NOT DISPLAY CSS CUSTOM PROPERTY NAMES AND THEREFORE DOESN'T RENDER
for (prop in elems) {
  const container = document.createElement('div');
  const label = document.createElement('label');
  const input = document.createElement('input');
  
  container.className = 'container';
  
  if (elems.hasOwnProperty(prop) && elems[prop].includes('--')) {
    label.setAttribute('for', elems[prop].replace('--', ''));
    label.innerHTML = elems[prop];
    input.setAttribute('id', elems[prop].replace('--', ''))
    input.setAttribute('type', 'text');
    input.value = elems.getPropertyValue(elems[prop]);

    const test = elems[prop]
    input.addEventListener('input', () => updateCustomProp(test, input.value));

    container.appendChild(label);
    container.appendChild(input);
    sidebar.appendChild(container);
  }
}

updateCustomProp = (propertyName, value) => {
  document.querySelector('some-elem').style.setProperty(propertyName, value);
}
              
            
!
999px

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