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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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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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You can apply a script from anywhere on the web to your Pen. Just put a URL to it here and we'll add it, in the order you have them, before the JavaScript in the Pen itself.

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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="wrapper">
  <section>
    <h2>Form Validation with HTML/CSS</h2>
    <form class="css-form">
      <input type="email" id="email" required placeholder="Email">
      <input type="password" id="password" minlength="8" required placeholder="Password">
      <button type="submit">Submit</button>
    </form>
  </section>

  <section>
    <h2>Form Validation with JavaScript</h2>
    <form id="js-form">
      <input id="email-js" placeholder="Email">
      <input id="password-js" placeholder="Password">
      <button id="submit-js">Submit</button>
    </form>
  </section>
</div>
!

CSS 

              
                .wrapper {
  display: flex;
  gap: 10px;
}

section {
  width: 50%;
  border: 1px solid grey;
  padding: 50px;
}

@media screen and (max-width: 960px) {
  .wrapper {
    flex-direction: column;
  }
  section {
    width: calc(100% - 100px);
  }
}

form {
  display: flex;
  flex-direction: column;
}

input {
  margin-bottom: 15px;
  padding: 10px;
}

.css-form input:valid {
  border: green 1px solid;
}

button {
  padding: 10px;
  background-color: #003366;
  color: white;
  border: none;
  cursor: pointer;
  width: 100%;
}
!

JS 

              
                document.getElementById("submit-js").addEventListener("click", () => {
  const email = document.getElementById("email-js").value;
  const password = document.getElementById("password-js").value;
  let valid = true;

  if (!email.includes("@")) {
    alert("Please enter a valid email");
    valid = false;
  }

  if (password.length < 8) {
    alert("Password must be at least 8 characters long");
    valid = false;
  }

  if (valid) {
    alert("Form submitted successfully!");
  }
});
!
999px

Console