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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

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About the <head>

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Insecure Resource

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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 

              
                <html>
  <head>
    <title>Toggle Popover Tutorial</title>
  </head>
  <body>
    <section class="title-section"><h1>Click Button to Open Popover. Then scroll and adjust browser width to see the positioning magic!</h1></section>
    <section id="popper-section">
    <button id="popper-button">Toggle Popover</button>
    <div id="popper-popup">
      <a href="#">menu item</a>
      <a href="#">menu item</a>
      <a href="#">menu item</a>
      <a href="#">menu item</a>
      <div id="popper-arrow"></div>
    </div>
      
    </section>
     <script src="https://unpkg.com/@popperjs/core@2"></script>
  </body>
!

CSS 

              
                * {
  box-sizing: border-box;
}

body {
  padding: 0;
  margin: 95vh 0;
  font-family: "lato";
}
section {
  width: 100%;
  max-width: 800px;
  margin: 0 auto;
}
.title-section {
  display:block;
  max-width: 500px;
  padding: 3%;
  position: fixed;
  top: 50%;
  right: 0;
  z-index:-1;
}
button {
  font: 500 20px lato !important;
  color: #ffffff;
  border: none;
  background: purple;
  padding: 1em 2em;
}
button:hover {
  cursor:pointer;
}

#popper-popup {
  display: none;

  background: #333;
  color: white;
  font-weight: bold;
  padding: 20px;
  font-size: 13px;
  border-radius: 4px;
  display: none;
  width: 100%;
  max-width: 400px;
}
#popper-popup a {
  font-size: 16px;
  font-weight: 300;
  display: block;
  padding: 20px;
  color: #ffffff;
  text-align: center;
  text-decoration: none;
  text-transform: uppercase;
}
#popper-popup a:hover {
  background: #444444;
}
#popper-popup[data-popper-reference-hidden] {
  visibility: hidden;
  pointer-events: none;
}
#popper-popup[show-popper] {
  display: block;
  z-index: 9999;
}

/*keep popper visible in the Divi Builder for editing*/
#et-fb-app #popper-popup {
  display: block;
}

/*build the arrow targeting the divider with class 'popper-arrow'*/
#popper-arrow,
#popper-arrow::before {
  position: absolute;
  width: 8px;
  height: 8px;
  background: #333333;
}
#popper-arrow {
  visibility: hidden;
}
#popper-arrow::before {
  visibility: visible;
  content: "";
  transform: rotate(45deg);
}

/*position arrow using popper.js data-popper-placement attribute*/
#popper-popup[data-popper-placement^="top"] #popper-arrow {
  bottom: -4px;
}

#popper-popup[data-popper-placement^="bottom"] #popper-arrow {
  top: -4px;
}

#popper-popup[data-popper-placement^="left"] #popper-arrow {
  right: -4px;
}

#popper-popup[data-popper-placement^="right"] #popper-arrow {
  left: -4px;
}

/*hide arrow when reference outside of viewport*/
#popper-popup[data-popper-reference-hidden] #popper-arrow::before {
  visibility: hidden;
}
!

JS 

              
                const popperButton = document.querySelector("#popper-button");
const popperPopup = document.querySelector("#popper-popup");
const popperSection = document.querySelector("#popper-section");
const popperArrow = document.querySelector("#popper-arrow");

let popperInstance = null;

//create popper instance
function createInstance() {
  popperInstance = Popper.createPopper(popperButton, popperPopup, {
    placement: "auto", //preferred placement of popper
    modifiers: [
      {
        name: "offset", //offsets popper from the reference/button
        options: {
          offset: [0, 8]
        }
      },
      {
        name: "flip", //flips popper with allowed placements
        options: {
          allowedAutoPlacements: ["right", "left", "top", "bottom"],
          rootBoundary: "viewport"
        }
      }
    ]
  });
}

//destroy popper instance
function destroyInstance() {
  if (popperInstance) {
    popperInstance.destroy();
    popperInstance = null;
  }
}

//show and create popper
function showPopper() {
  popperPopup.setAttribute("show-popper", "");
  popperArrow.setAttribute("data-popper-arrow", "");
  createInstance();
}

//hide and destroy popper instance
function hidePopper() {
  popperPopup.removeAttribute("show-popper");
  popperArrow.removeAttribute("data-popper-arrow");
  destroyInstance();
}

//toggle show-popper attribute on popper to hide or show it with CSS
function togglePopper() {
  if (popperPopup.hasAttribute("show-popper")) {
    hidePopper();
  } else {
    showPopper();
  }
}
//execute togglePopper function when clicking the popper reference/button
popperButton.addEventListener("click", function (e) {
  e.preventDefault();
  togglePopper();
});
!
999px

Console