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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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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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Packages

Add Packages

Search for and use JavaScript packages from npm here. By selecting a package, an import statement will be added to the top of the JavaScript editor for this package.

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Using packages here is powered by esm.sh, which makes packages from npm not only available on a CDN, but prepares them for native JavaScript ESM usage.

All packages are different, so refer to their docs for how they work.

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HTML 

              
                <div class="container">
  <div id="timer">00:00</div>
  <button onclick="startTimer()">Start</button>
</div>
!

CSS 

              
                * {
  margin: 0;
  padding: 0;
  font-family: Roboto, "Helvetica Neue", Arial, sans-serif;
}
.container {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
  flex-direction: column;
  background: #e9c46a;
}
#timer {
  margin-bottom: 25px;
  font-size: 3rem;
  font-weight: bold;
  color: #2a9d8f;
  &.odd {
    color: #264653;
  }
}
button {
  border-radius: 5px;
  display: inline-block;
  border: none;
  padding: 1rem 2rem;
  margin: 0;
  text-decoration: none;
  background: #e76f51;
  color: #ffffff;
  font-family: sans-serif;
  font-size: 1rem;
  cursor: pointer;
  text-align: center;
  transition: background 250ms ease-in-out;
  -webkit-appearance: none;
  -moz-appearance: none;
  &:hover {
    background: #f4a261;
  }
}
!

JS 

              
                const timer = document.getElementById("timer");
let timerInterval;

startTimer = () => {
  // Firs twe start by clearing the existing timer, in case of a restart
  clearInterval(timerInterval);
  // Then we clear the variables
  let second = 0,
    minute = 0,
    hour = 0;

  // Next we set a interval every 1000 ms
  timerInterval = setInterval(function () {
    // Toggle the odd class every interval
timer.classList.toggle('odd');

    // We set the timer text to include a two digit representation
    timer.innerHTML =
      (hour ? hour + ":" : "") +
      (minute < 10 ? "0" + minute : minute) +
      ":" +
      (second < 10 ? "0" + second : second);

    // Next we add a new second since one second is passed
    second++;

    // We check if the second equals 60 "one minute"
    if (second == 60) {
      // If so, we add a minute and reset our seconds to 0
      minute++;
      second = 0;
    }

    // If we hit 60 minutes "one hour" we reset the minutes and plus an hour
    if (minute == 60) {
      hour++;
      minute = 0;
    }
  }, 1000);
};
!
999px

Console