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HTML

CSS

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Add External Stylesheets/Pens

Any URL's added here will be added as <link>s in order, and before the CSS in the editor. If you link to another Pen, it will include the CSS from that Pen. If the preprocessor matches, it will attempt to combine them before processing.

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JavaScript

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HTML

              
                <details>
  <summary>
    <span data-css-icon="down">Down To Up<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus">Plus To Minus<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right">Right To Down<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross">Plus to Cross<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals">Equals to Cross<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="down outline">Down To Up: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus outline">Plus To Minus: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right outline">Right To Down: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross outline">Plus to Cross: Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals outline">Equals to Cross: Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="down fill">Down To Up: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus fill">Plus To Minus: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right fill">Right To Down: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross fill">Plus to Cross: Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals fill">Equals to Cross: Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square down outline">Down To Up: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square plus outline">Plus To Minus: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square right outline">Right To Down: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square cross outline">Plus to Cross: Square Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square equals outline">Equals to Cross: Square Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square down fill">Down To Up: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square plus fill">Plus To Minus: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square right fill">Right To Down: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square cross fill">Plus to Cross: Square Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square equals fill">Equals to Cross: Square Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae</div>
</details>

<h2>Menu: Requires extra element for outline/fill</h2>
<details>
  <summary>
    <span data-css-icon="menu">Menu to Cross<i></i></span>
  </summary>
 <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<div dir="rtl">
  <h2> right-to-left</h2>
 <details>
  <summary>
    <span data-css-icon="down">Down To Up<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus">Plus To Minus<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right">Right To Down<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross">Plus to Cross<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals">Equals to Cross<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="down outline">Down To Up: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus outline">Plus To Minus: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right outline">Right To Down: Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross outline">Plus to Cross: Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals outline">Equals to Cross: Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="down fill">Down To Up: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="plus fill">Plus To Minus: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="right fill">Right To Down: Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="cross fill">Plus to Cross: Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="equals fill">Equals to Cross: Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square down outline">Down To Up: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square plus outline">Plus To Minus: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square right outline">Right To Down: Square Outline<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square cross outline">Plus to Cross: Square Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square equals outline">Equals to Cross: Square Outline<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square down fill">Down To Up: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eos minus quo ipsam culpa! Dicta, reiciendis.
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square plus fill">Plus To Minus: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor sit amet consectetur adipisicing elit. Ipsam commodi amet fuga dicta facilis! Sed ipsum unde cumque debitis omnis placeat non, ut magni dolores!
  </div>
</details>

<details>
  <summary>
    <span data-css-icon="square right fill">Right To Down: Square Fill<i></i></span>
  </summary>
  <div>Lorem ipsum dolor, sit amet consectetur adipisicing elit. Odit, quam. Ipsum deserunt veniam placeat iusto vel possimus omnis quo dolor distinctio fugit. Ex, impedit est?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square cross fill">Plus to Cross: Square Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>

<details>
  <summary>
    <span data-css-icon="square equals fill">Equals to Cross: Square Fill<i></i></span>
  </summary>
  <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae</div>
</details>

<h2>Menu: Requires extra element for outline/fill</h2>
<details>
  <summary>
    <span data-css-icon="menu">Menu to Cross<i></i></span>
  </summary>
 <div>Lorem, ipsum dolor sit amet consectetur adipisicing elit. Numquam ab animi velit est unde, quod, debitis voluptas, tenetur possimus corporis veritatis commodi eveniet placeat officiis non! Et consequatur nulla molestiae?</div>
</details>
</div>
              
            
!

CSS

              
                /*
For further documentation about CSS Icons, see this pen:
https://codepen.io/stoumann/pen/eYzPVqe
*/

[data-css-icon] {
	--animdur: .3s;
  --loading-animdur: 0.8s;
	--animtf: ease-in;
	--bdw: 2px;
  --bdrs: 50%;
  --bgc: transparent;
	--c: currentcolor;
  --dots-bgc: silver;
  --dots-size: 0.5rem;
  --icon-size: 1rem;
	--size: 2.5rem;

	align-items: center;
  cursor: pointer;
	display: flex;
	justify-content: space-between;
}
[data-css-icon] i {
  align-items: center;
  background-color: var(--bgc);
  border-radius: var(--bdrs);
  box-sizing: border-box;
  display: inline-flex;
  height: var(--size);
  justify-content: center;
  position: relative;
  transition: background-color var(--animdur) var(--animtf);
  width: var(--size);
}
[data-css-icon] i::after,
[data-css-icon] i::before {
  transform-origin: 50% 50%;
	transition: all var(--animdur) var(--animtf);
}

[data-css-icon*="down"] i::after,
[data-css-icon*="right"] i::after {
  background: transparent;
	border-color: var(--c);
	border-style: solid;
  box-sizing: border-box;
	content: '';
	display: inline-block;
	height: var(--icon-size) ;
	margin: 0;
  position: relative;
	width: var(--icon-size);
}
[data-css-icon*="down"] i::after {
	border-width: 0 var(--bdw) var(--bdw) 0;
  top: calc(0px - (var(--icon-size) / 4));
  transform: rotate(45deg);
}
[data-css-icon*="right"] i::after {
	border-width: var(--bdw) var(--bdw) 0 0;
  right: calc((var(--icon-size) / 4));
  top: 0;
	transform: rotate(45deg);
}

[data-css-icon*="equals"] i::after,
[data-css-icon*="equals"] i::before,
[data-css-icon*="cross"] i::after,
[data-css-icon*="cross"] i::before,
[data-css-icon*="menu"] i,
[data-css-icon*="menu"] i::after,
[data-css-icon*="menu"] i::before,
[data-css-icon*="plus"] i::after,
[data-css-icon*="plus"] i::before {
  /* Width need to be the diagonal of the down-arrow side-length (--size): sqrt(2) * --size. */
	--w: calc(var(--icon-size) * 1.4142135623730950488016887242097);
	background: var(--c);
	content: '';
	height: var(--bdw);
	position: absolute;
	width: var(--w);
}
[data-css-icon*="cross"] i::before,
[data-css-icon*="plus"] i::before {
	transform: rotate(90deg);
}
[data-css-icon*="equals"] i {
  --m: 4px;
}
[data-css-icon*="equals"] i::after {
  transform: translateY(var(--m));
}
[data-css-icon*="equals"] i::before {
  transform: translateY(calc(0px - var(--m)));
}
[data-css-icon*="dots"],
[data-css-icon*="menu"] {
  height: var(--size);
}
[data-css-icon*="menu"] i {
  --bdrs: 0;
  --m: 7px;
  position: relative;
  right: calc((var(--size) - var(--w)) / 2);
}
[data-css-icon*="menu"] i::after {
  top: var(--m);
}
[data-css-icon*="menu"] i::before {
  top: calc(0px - var(--m));
}

[data-css-icon*="dots"] i,
[data-css-icon*="dots"] i::after,
[data-css-icon*="dots"] i::before {
  animation: dots var(--loading-animdur) infinite alternate;
  background-color: var(--c);
  border-radius: 50%;
  content: "";
  display: inline-block;
  height: var(--dots-size);
  width: var(--dots-size);
}
[data-css-icon*="dots"] i {
  animation-delay: var(--loading-animdur);
  position: relative;
  right: calc((var(--size) - var(--dots-size)) / 4);
}
[data-css-icon*="dots"] i::after {
  animation-delay: 0s;
  left: calc(0px - (var(--dots-size) * 3));
  position: absolute;
}
[data-css-icon*="dots"] i::before {
  animation-delay: calc(var(--loading-animdur) / 2);
  left: calc(0px - (var(--dots-size) * 1.5));
  position: absolute;
}

[data-css-icon*="spin"] i::after {
  animation: spin var(--loading-animdur) infinite linear;
  border-radius: 50%;
  border: var(--bdw) solid var(--dots-bgc);
  border-left: var(--bdw) solid var(--c);
  content: "";
  height: var(--icon-size);
  transform: translateZ(0);
  width: var(--icon-size);
}

/* STATE */
[open] > summary > [data-css-icon*="cross"] i::after {
	transform: rotate(45deg);
}
[open] > summary > [data-css-icon*="cross"] i::before {
  transform: rotate(135deg);
}
[open] > summary > [data-css-icon*="down"] i::after {
  top: var(--bdw);
	transform: rotate(45deg) scale(-1) ;
}
[open] > summary > [data-css-icon*="right"] i::after {
  right: 0;
  top: calc(0px - (var(--icon-size) / 4));
	transform: rotate(135deg);
}
[open] > summary > [data-css-icon*="plus"] i::after {
	transform: rotate(180deg);
}
[open] > summary > [data-css-icon*="plus"] i::before {
	transform: rotate(-0deg);
}

[open] > summary > [data-css-icon*="equals"] i::after { transform: rotate(-45deg); }
[open] > summary > [data-css-icon*="equals"] i::before { transform: rotate(45deg); }

[open] > summary > [data-css-icon*="menu"] i { background-color: transparent; }
[open] > summary > [data-css-icon*="menu"] i::after { transform: translateY(calc(0px - var(--m))) rotate(-45deg); }
[open] > summary > [data-css-icon*="menu"] i::before { transform: translateY(var(--m)) rotate(45deg); }

/* RTL: inset-block-end fails */
[dir="rtl"] [data-css-icon*="right"] i::after {
	left: calc((var(--icon-size) / 4));
}
[dir="rtl"] [data-css-icon*="menu"] i {
	left: 0.5rem;
  right: auto;
}

/* MODIFIERS */
[data-css-icon*="outline"] i {
  border: var(--bdw) solid var(--c);
}
[data-css-icon*="fill"] {
  --bgc: hsl(195, 10%, 30%);
  --c: hsl(195, 10%, 95%);
}
[data-css-icon*="square"] {
  --bdrs: 5px;
}

/* For this demo only */
body { font-family: system-ui, sans-serif; padding: 1rem; }
button, details { width: 25rem; }
button { font-family: inherit; font-size: inherit; }
details {
  height: var(--collapsed);
  overflow: hidden;
  transition: height 300ms cubic-bezier(0.4, 0.01, 0.165, 0.99);
}
details[open] {
  height: var(--expanded);
}
button,
summary { 
  background-color: var(--bgc);
  border: 0;
  border-radius: 5px;
  color: var(--c, inherit);
  list-style-type: none;
  margin: 0.5rem 0;
  outline: none;
  padding-bottom: 0.5rem;
  padding-top: 0.5rem;
  padding-inline-end: 0.5rem;
  padding-inline-start: 1rem;
  user-select: none;
}
summary::marker { display: none; }
summary::-webkit-details-marker { display: none; }

/* 4 States of summary */
button,
summary {
  --bgc: hsl(195, 10%, 90%);
}
[open] > summary {
  --bgc: hsl(195, 10%, 20%);
  --c: hsl(195, 10%, 92%);
}
button:focus,
summary:focus {
  --bgc: hsl(195, 10%, 75%);
}
[open] > summary:focus {
  --bgc: hsl(195, 10%, 10%);
    --c: hsl(195, 10%, 99%);
}
[open] > summary:focus > [data-css-icon*="fill"],
[open] > summary > [data-css-icon*="fill"] {
  --bgc: hsl(195, 10%, 80%);
  --c: hsl(195, 10%, 10%);
}
summary + * {
  color: #777;
  line-height: 1.6;
  padding: 0.5rem;
}

@keyframes dots {
  0% {
    background-color: var(--c);
  }
  50%,
  100% {
    background-color: var(--dots-bgc);
  }
}

@keyframes spin {
  0% {
    transform: rotate(0deg);
  }
  to {
    transform: rotate(1turn);
  }
}
              
            
!

JS

              
                function setDetailsHeight(selector, wrapper = document) {
	const setHeight = (detail, open = false) => {
		detail.open = open;
		const rect = detail.getBoundingClientRect();
		detail.dataset.width = rect.width;
		detail.style.setProperty(open ? `--expanded` : `--collapsed`,`${rect.height}px`);
	}
	const details = wrapper.querySelectorAll(selector);
	const RO = new ResizeObserver(entries => {
		return entries.forEach(entry => {
			const detail = entry.target;
			const width = parseInt(detail.dataset.width, 10);
			if (width !== entry.contentRect.width) {
				detail.removeAttribute('style');
				setHeight(detail);
				setHeight(detail, true);
				detail.open = false;
			}
		})
	});
	details.forEach(detail => {
		RO.observe(detail);
	});
}

/* Run it */
setDetailsHeight('details');
              
            
!
999px

Console