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HTML 

              
                <!DOCTYPE html>
<html lang="en">
  <head>
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Decimal-Binary Converter</title>
    <!-- Design by groundttorial.com -->
    <link
      href="https://fonts.googleapis.com/css2?family=Poppins:wght@400;500&display=swap"
      rel="stylesheet"
    />
    <!-- Stylesheet -->
    <style>
      * {
  padding: 0;
  margin: 0;
  box-sizing: border-box;
  font-family: "Poppins", sans-serif;
}
body {
  background-color: #0075ff;
}
.container {
  background-color: #ffffff;
  width: 80vmin;
  min-width: 480px;
  max-width: 37.5em;
  padding: 3em 2.5em;
  position: absolute;
  transform: translate(-50%, -50%);
  top: 50%;
  left: 50%;
  border-radius: 0.62em;
  box-shadow: 0 1.2em 2.5em rgba(0, 21, 44, 0.18);
}
h2 {
  font-size: 1.8em;
  color: #0075ff;
  text-align: center;
  font-weight: 600;
  letter-spacing: 0.5px;
  margin-bottom: 1.3em;
}
.wrapper {
  display: flex;
  justify-content: space-between;
  gap: 1.8em;
}
label {
  display: block;
  margin-bottom: 0.5em;
  font-weight: 500;
  color: #050121;
}
input {
  position: relative;
  font-size: 1.1em;
  width: 100%;
  padding: 0.5em;
  border-radius: 0.2em;
  border: 1.5px solid #43405c;
  color: #43405c;
  outline: none;
}
input:focus {
  border-color: #0075ff;
}
#error-msg {
  text-align: center;
  margin-top: 1.25em;
  color: #ff4362;
}
    </style>
  </head>
  <body>
    <div class="container">
      <h2>Decimal Binary Converter</h2>
      <div class="wrapper">
        <div class="input-wrapper">
          <label for="dec-inp">Decimal:</label>
          <input type="number" id="dec-inp" />
        </div>
        <div class="input-wrapper">
          <label for="bin-inp">Binary:</label>
          <input type="number" id="bin-inp" />
        </div>
      </div>
      <p id="error-msg"></p>
    </div>
    <!-- Script -->
    <script>
      //Initial References
let decInp = document.getElementById("dec-inp");
let binInp = document.getElementById("bin-inp");
let errorMsg = document.getElementById("error-msg");

//Convert decimal to binary when user inputs in the decimal field
decInp.addEventListener("input", () => {
  let decValue = parseInt(decInp.value);
  //Converts the decimal value to binary
  binInp.value = decValue.toString(2);
});

//Convert binary to decimal when user inputs in the binary field
binInp.addEventListener("input", () => {
  let binValue = binInp.value;
  //If the binary number is valid convert it to decimal
  if (binValidator(binValue)) {
    decInp.value = parseInt(binValue, 2);
    errorMsg.textContent = "";
  }
  //Else display an error message
  else {
    errorMsg.textContent = "Please Enter An Valid Binary Input";
  }

  //Function to check if the binary number is valid i.e it does not contain any number other than 0 and 1
  function binValidator(num) {
    for (let i = 0; i < num.length; i++) {
      if (num[i] != "0" && num[i] != "1") {
        return false;
      }
    }
    return true;
  }
});
    </script>
  </body>
</html>
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