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HTML

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HTML preprocessors can make writing HTML more powerful or convenient. For instance, Markdown is designed to be easier to write and read for text documents and you could write a loop in Pug.

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Adding Classes

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. So you don't have access to higher-up elements like the <html> tag. If you want to add classes there that can affect the whole document, this is the place to do it.

About the <head>

In CodePen, whatever you write in the HTML editor is what goes within the <body> tags in a basic HTML5 template. If you need things in the <head> of the document, put that code here.

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Insecure Resource

The resource you are linking to is using the 'http' protocol, which may not work when the browser is using https.

CSS

About CSS Preprocessors

CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

It's a common practice to apply CSS to a page that styles elements such that they are consistent across all browsers. We offer two of the most popular choices: normalize.css and a reset. Or, choose Neither and nothing will be applied.

Vendor Prefixing

About Vendor Prefixing

To get the best cross-browser support, it is a common practice to apply vendor prefixes to CSS properties and values that require them to work. For instance -webkit- or -moz-.

We offer two popular choices: Autoprefixer (which processes your CSS server-side) and -prefix-free (which applies prefixes via a script, client-side).

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JavaScript

About JavaScript Preprocessors

JavaScript preprocessors can help make authoring JavaScript easier and more convenient.

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Babel includes JSX processing.

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HTML 

              
                <accordion-wrapper>
  <a-section>
    <a-title>
      <span>Paris</span><span>🧀</span>
    </a-title>
    <a-content>
      Paris is the capital and most populous city of France, with a population of 2,148,271 residents. Since the 17th century, Paris has been one of Europe's major centres of finance, diplomacy, commerce, fashion, science and the arts.
    </a-content>
  </a-section>
  <a-section>
    <a-title>
      <span>Lech</span><span>⛷</span>
    </a-title>
    <a-content>
      Lech am Arlberg is a mountain village and an exclusive ski resort in the Bludenz district in the Austrian state of Vorarlberg on the banks of the river Lech.
    </a-content>
  </a-section>
  <a-section>
    <a-title>
      <span>Madrid</span><span>🍷</span>
    </a-title>
    <a-content>
      Madrid is the capital and most populous city of Spain. As the capital city of Spain, seat of government, and residence of the Spanish monarch, Madrid is also the political, economic and cultural centre of the country.
    </a-content>
  </a-section>
</accordion-wrapper>

<accordion-wrapper class="sideways">
  <a-section>
    <a-title>
      <span>Paris</span><span>🧀</span>
    </a-title>
    <a-content>
      Paris is the capital and most populous city of France, with a population of 2,148,271 residents. Since the 17th century, Paris has been one of Europe's major centres of finance, diplomacy, commerce, fashion, science and the arts.
    </a-content>
  </a-section>
  <a-section>
    <a-title>
      <span>Lech</span><span>⛷</span>
    </a-title>
    <a-content>
      Lech am Arlberg is a mountain village and an exclusive ski resort in the Bludenz district in the Austrian state of Vorarlberg on the banks of the river Lech.
    </a-content>
  </a-section>
  <a-section>
    <a-title>
      <span>Madrid</span><span>🍷</span>
    </a-title>
    <a-content>
      Madrid is the capital and most populous city of Spain. As the capital city of Spain, seat of government, and residence of the Spanish monarch, Madrid is also the political, economic and cultural centre of the country.
    </a-content>
  </a-section>
</accordion-wrapper>
!

CSS 

              
                accordion-wrapper {
  display: block;
  margin: 30px auto;
  width: 400px;
  box-shadow: 0 10px 20px -10px #646f74;
}

a-title::part(link) {
  display: flex;
  justify-content: space-between;
  padding: 15px 15px 15px 25px;
  background: white;
  border-top: 1px solid #edf2f8;
  border-bottom: 1px solid white;
  cursor: pointer;
  transition: border 0.2s, font-weight 0.2s;
}
a-title::part(link)::before {
  display: inline;
  content: "+";
}
a-title::part(link):hover {
  border-bottom: 1px solid #646f74;
  font-weight: bold;
}
a-title::part(expanded) {
  border-bottom: 1px solid #646f74;
}
a-title::part(expanded)::before {
  content: "-";
}
a-content::part(content) {
  background: #edf2f8;
  visibility: hidden;
  height: 0;
  padding: 0;
  font-size: 0;
  transition: height 0.2s, visibility 0.2s, padding 0.2s;
}
a-content::part(content expanded) {
  visibility: visible;
  height: auto;
  padding: 15px;
  font-size: 1em;
  border-bottom: 1px solid #edf2f8;
}

accordion-wrapper.sideways {
  display: flex;
  flex-flow: row nowrap;
  // width: 100%;
  // max-width: 1920px;

  a-section {
    display: flex;
    flex-flow: row nowrap;
    align-items: stretch;
  }

  a-title::part(link) {
    flex-flow: column nowrap;
    justify-content: center;
    align-items: center;
    height: 100%;
    box-sizing: border-box;
  }

  a-title::part(link):hover {
    border-right: 1px solid #646f74;
    border-bottom: none;
    font-weight: bold;
  }
  a-title::part(expanded) {
    border-right: 1px solid #646f74;
    border-bottom: none;
  }

  a-content::part(content) {
    height: auto;
    width: 0;
    transition: width 0.2s, visibility 0.2s, padding 0.2s;
  }
  a-content::part(content expanded) {
    width: auto;
  }
}
!

JS 

              
                import {
  ref,
  provide,
  inject,
  computed,
  defineCustomElement
} from "https://unpkg.com/vue@3/dist/vue.esm-browser.js";

const AccordionWrapper = defineCustomElement({
  props: ["class"],
  template: `<slot />`,
  setup() {
    const activeIndex = ref(-1);
    provide("activeIndex", activeIndex);
    return {
      activeIndex
    };
  }
});

customElements.define("accordion-wrapper", AccordionWrapper);

let SECTION_INDEX = 0;
const ASection = defineCustomElement({
  props: ["class"],
  template: `<slot />`,
  setup() {
    const activeIndex = inject("activeIndex");

    // provide
    const index = SECTION_INDEX;
    provide("index", index);
    SECTION_INDEX++;

    if (activeIndex.value === -1) {
      activeIndex.value = index;
    }

    provide("activate", () => {
      activeIndex.value = index;
    });

    provide(
      "isActive",
      computed(() => activeIndex.value === index)
    );

    return {
      index
    };
  }
});

customElements.define("a-section", ASection);

const ATitle = defineCustomElement({
  props: ["class"],
  template: `<a :part="classes + ' link'" @click="activate"><slot/></a>`,
  setup() {
    const isActive = inject("isActive");
    const activate = inject("activate");

    const classes = computed(() => {
      return isActive.value ? "expanded" : "closed";
    });

    return {
      activate,
      classes
    };
  }
});

customElements.define("a-title", ATitle);

const AContent = defineCustomElement({
  props: ["class"],
  template: `<div :part="classes + ' content'"><slot /></div>`,
  setup() {
    const isActive = inject("isActive");

    const classes = computed(() => {
      return isActive.value ? "expanded" : "closed";
    });

    return {
      classes
    };
  }
});

customElements.define("a-content", AContent);
!
999px

Console