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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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About the <head>

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!
Insecure Resource

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CSS

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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.

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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-.

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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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Packages

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HTML 

              
                <!-- https://stackoverflow.com/questions/50538768/css-custom-cursor-that-changes-depending-on-hovered-element-flickers-when-movi -->
<div id="wrapper">
  <div id="box-1" class="box">sphere</div>
  <div id="box-2" class="box">circle outline</div>
  <div id="box-3" class="box">circle pin</div>
  <div id="box-4" class="box">circle color gradient</div>
  <div id="circle"></div>
</div>

<div class="citation">
  <p>Recreation of the solution <a class="cite-link" href="https://stackoverflow.com/questions/50538768/css-custom-cursor-that-changes-depending-on-hovered-element-flickers-when-movi" target="_blank">here</a> for my own edification.</p>
</div>
!

CSS 

              
                html {
  box-sizing: border-box;
}

*, *::before, *::after {
  box-sizing: inherit;
  &:focus {
    outline: none;
  }
}

* {
  font-family: monaco, courier;
}

//

body {
  position: relative;
  margin: 0;
  height: 100vh;
  background-color: #acd1d2;
  display: flex;
  justify-content: center;
  align-items: center;
  font-family: monospace;
  cursor: none;
}

#wrapper {
  position: relative;
  width: 70%;
  height: 80%;
}

#circle {
  position: fixed;
  z-index: 5;
  width: 32px;
  height: 32px;
  background-color: white;
  border-radius: 50%;
  pointer-events: none;
  transition:
    background ease-in 10ms,
    box-shadow ease-in 150ms,
    transform ease-in 150ms;
  /* promote it to its own layer to enable  hardware accelerated rendering: */
  transform: translate3d(0, 0, 0);
}

.box {
  height: 25%;
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;  
}

#box-1 {
  background-color: #e8edf3;
}
  
#box-1:hover ~ #circle {
  background-color: #e6cf8b;
  box-shadow: 0 0 0 0 transparent, inset 0em -0.3em 0.4em 0.2em #ca9e03a6;
}

#box-2 {
  background-color: #e6cf8b;
}
  
#box-2:hover ~ #circle {
  background-color: transparent;
  /* use box-shadow instead of border to avoid changing the dimensions of the cursor, which would force it off-center until the mouse moves again: */
  box-shadow: 0 0 0 3px #E91E63;
}

#box-3 {
  background-color: #b56969; 
}
  
#box-3:hover ~ #circle {
  /* change size with scale() instead of width and height for better performance: */
  transform: scale(0.5) translate3d(0, 0, 0);
  background-color: #e6cf8b;
} 

#box-4 {
  color: white;
  background-color: #22264b;
}

#box-4:hover ~ #circle {
  background-image: linear-gradient(to top, #fbc2eb 0%, #a6c1ee 100%);
}

// citation

.citation {
  position: fixed;
  bottom: 0;
  left: 0;
  z-index: 100;
  background: #91b9ba;
  cursor: default;
  p {
    margin: 0;
    padding: 10px;
    font-size: 12px;
    color: #063738;
    a {
      color: DarkBlue;
      cursor: pointer;
      &:hover {
        color: Indigo;
      }
    }
  }
}
!

JS 

              
                const circle = document.getElementById('circle');
const circleStyle = circle.style;

document.addEventListener('mousemove', e => {
  window.requestAnimationFrame(() => {
    circleStyle.top = `${ e.clientY - circle.offsetHeight/2 }px`;
    circleStyle.left = `${ e.clientX - circle.offsetWidth/2 }px`;
  });
});
!
999px

Console