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CSS

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HTML 

              
                <div class="calculator">
      <div class="screen mb16">
        <div class="result">0</div>
      </div>
      <div class="buttons">
        <button data-digit="7" class="btn btn-primary blue">7</button>
        <button data-digit="8" class="btn btn-primary blue">8</button>
        <button data-digit="9" class="btn btn-primary blue">9</button>
        <button data-operation="+" class="btn btn-primary red">+</button>

        <button data-digit="4" class="btn btn-primary blue">4</button>
        <button data-digit="5" class="btn btn-primary blue">5</button>
        <button data-digit="6" class="btn btn-primary blue">6</button>
        <button data-operation="-" class="btn btn-primary red">-</button>

        <button data-digit="1" class="btn btn-primary blue">1</button>
        <button data-digit="2" class="btn btn-primary blue">2</button>
        <button data-digit="3" class="btn btn-primary blue">3</button>
        <button data-operation="*" class="btn btn-primary red">*</button>

        <button data-digit="0" class="btn btn-primary blue">0</button>
        <button data-clear class="btn btn-primary red-dark">C</button>
        <button data-equals class="btn btn-primary red">=</button>
        <button data-operation="/" class="btn btn-primary red">/</button>
      </div>
    </div>

    <!-- made in jssecrets.com -->
    <a class="jssecrets" href="https://jssecrets.com/" target="_blank"
      >jssecrets.com</a
    >
    <!-- made in jssecrets.com -->
!

CSS 

              
                @import url("https://fonts.googleapis.com/css2?family=Urbanist:wght@500&display=swap");

*,
::before,
::after {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

ul {
  list-style: none;
}

.mb16 {
  margin-bottom: 16px;
}

@font-face {
  font-family: "Windows 95";
  src: url('https://ilyass.sg-host.com/w95fa.woff2') format('woff2');
  font-weight: normal;
  font-style: normal;
}
.red {
  color: #ff0000;
}

.blue {
  color: #0000ff;
}

.red-dark {
  color: #800000;
}

* {
  font-family: "Windows 95", sans-serif;
}

body {
  height: 100vh;
  background: #eaeaea;
  position: relative;
  display: flex;
  justify-content: center;
  align-items: center;
  overflow: hidden;
}

.calculator {
  padding: 24px;
  background: #c0c0c0;
  border-width: 3px;
  border-style: solid;
  border-right-color: #606f6f;
  border-bottom-color: #606f6f;
  border-top-color: #f1f1f1;
  border-left-color: #f1f1f1;
  /* max-width: 427.5px; */
}
@media (max-width: 480px) {
  .calculator {
    padding: 12px;
    max-width: 240px;
  }
}
.calculator .screen .result {
  width: 100%;
  max-width: 100%;
  height: 68px;
  text-align: right;
  padding: 16px;
  font-size: 32px;
  font-weight: bold;
  color: #000;
  background-color: #fff;
  border-width: 3px;
  border-style: solid;
  border-right-color: #f1f1f1;
  border-bottom-color: #f1f1f1;
  border-top-color: #606f6f;
  border-left-color: #606f6f;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}
@media (max-width: 480px) {
  .calculator .screen .result {
    font-size: 24px;
    height: 48px;
    line-height: 16px;
  }
}
.calculator .buttons {
  display: grid;
  gap: 8px;
  grid-template-columns: repeat(4, 1fr);
  grid-template-rows: repeat(4, 1fr);
}
.calculator .buttons button {
  font-size: 32px;
  font-weight: bold;
}
.calculator .buttons button:hover {
  cursor: pointer;
}
@media (max-width: 480px) {
  .calculator .buttons button {
    font-size: 20px;
  }
}

.btn {
  position: relative;
  transition: all 0.2s ease;
  padding: 0.7rem 2.1rem;
}
@media (max-width: 480px) {
  .btn {
    padding: 0.35rem 1rem;
  }
}

.btn:before,
.btn:after {
  content: "";
  position: absolute;
  box-sizing: border-box;
  display: block;
  background: transparent;
  z-index: 9;
  top: 0;
  left: 0;
}

.btn:before {
  width: 100%;
  height: 100%;
  border-top: 2px solid #f1f1f1;
  border-left: 2px solid #f1f1f1;
  border-right: 2px solid #606f6f;
  border-bottom: 2px solid #606f6f;
}

.btn:not(:disabled):not(.disabled):active:before,
.btn:not(:disabled):not(.disabled):active:focus:before {
  border-color: transparent;
}

.btn-primary {
  background-color: #c0c0c0;
  border-color: #a0a0a0;
}

.btn-primary:hover {
  background-color: #c0c0c0;
  border-color: #a0a0a0;
}

.btn-primary:focus,
.btn-primary.focus {
  box-shadow: none;
}

.jssecrets {
  font-weight: 500;
  text-decoration: none !important;
  font-family: "Urbanist";
  padding: 16px;
  background: #ffc805;
  color: #000000;
  font-size: 1.2em;
  letter-spacing: 0.2px;
  border-radius: 8px;
  box-shadow: 0px 4px 12px rgba(184, 144, 0, 0.1882352941), 0px 8px 24px rgba(184, 144, 0, 0.3764705882);
  position: absolute;
  bottom: 24px;
  right: 24px;
  transition: all 0.3s;
}
.jssecrets:hover {
  transform: translateY(-4px);
}
!

JS 

              
                // Variables
let a = '';
let b = '';
let currentOperation = '';
let isCompleted = false;

const result = document.querySelector('.result');
const digits = document.querySelectorAll('[data-digit]');
const operations = document.querySelectorAll('[data-operation]');
const equalsBtn = document.querySelector('[data-equals]');
const clearBtn = document.querySelector('[data-clear]');

// Functions

// function for edge cases
const edgeCase = (message) => {
  result.textContent = message;
  a = '';
  b = '';
  currentOperation = '';
  isCompleted = false;
};

const clearAll = () => {
  edgeCase('0');
};

const cleanScreen = () => {
  result.textContent = '';
};

const equalsFunction = () => {
  if (b === '') b = a;

  switch (currentOperation) {
    case '+':
      a = parseInt(a, 10) + parseInt(b, 10);
      break;
    case '-':
      a = parseInt(a, 10) - parseInt(b, 10);
      break;
    case '*':
      a = parseInt(a, 10) * parseInt(b, 10);
      break;
    case '/':
      if (b === '0') {
        edgeCase('Error');
        return;
      }
      a = parseInt(a, 10) / parseInt(b, 10);
      break;
  }
  isCompleted = true;

  // overflow check
  if (a >= 1e20 || a <= -1e20) {
    edgeCase('Overflow');
    a = 0;
    return;
  }

  result.textContent = a;

  // non-standard behaviour
  if (a === '' && b === '') {
    edgeCase('0');
  }
};

const enterOperation = (event) => {
  cleanScreen();

  // if currentOperation entered before first number
  if (a === '' && b === '') {
    a = 0;
  }

  // if equals btn is omited and operation btn used
  if (a !== '' && b !== '' && currentOperation !== '' && !isCompleted) {
    equalsFunction();
    currentOperation = event.currentTarget.dataset.operation;
    return;
  }

  currentOperation = event.currentTarget.dataset.operation;
  result.textContent = currentOperation;
};

const enterDigit = (event) => {
  //
  let symbol = event.currentTarget.dataset.digit;

  cleanScreen();

  if (b === '' && currentOperation === '') {
    a += symbol;
    result.textContent = parseInt(a, 10);

    if (a >= 1e20 || a <= -1e20) {
      result.textContent = 'Overflow';
      a = '';
      return;
    }
  }
  /////////////////
  else if (a !== '' && b !== '' && isCompleted) {
    b = symbol;
    isCompleted = false;
    result.textContent = parseInt(b, 10);
  }
  /////////////////
  else {
    b += symbol;
    result.textContent = parseInt(b, 10);

    if (b >= 1e20 || b <= -1e20) {
      result.textContent = 'Overflow';
      b = '';
      return;
    }
  }
};

// Event Listeners
clearBtn.addEventListener('click', clearAll);

digits.forEach((number) => {
  number.addEventListener('click', enterDigit);
});

operations.forEach((operation) => {
  operation.addEventListener('click', enterOperation);
});

equalsBtn.addEventListener('click', equalsFunction);
!
999px

Console