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HTML 

              
                <div id="top-half">
  <form id="top-half-content">
    <label for="arabic-entry">Insert Arabic numeral to convert:</label>
    <input id="arabic-entry" type="text" name="arabic-entry" placeholder="Ex: 1004">
  </form>
</div>

<div id="bottom-half">
  <p id="output">Nothing's been entered...</p>
</div>
!

CSS 

              
                /*Variables*/
$primary-default: #152B42;
$primary-light: #1159A6;
$primary-dark: #152B42;

$primary-default-text: white;
$primary-light-text: black;
$primary-dark-text: white;

$font-family: 'Kufam', cursive;

/*Styles*/
body {
  font-family: $font-family;
  margin: 0;
  padding: 0;
}

#top-half {
  //Layout
  position: relative;
  width: 100vw;
  height: 50vh;
  min-height: 200px;
  
  //Text
  line-height: 50vh;
  
  //Design
  color: $primary-default-text;
  background-color: $primary-default;
  
  form {
    //Layout
    position: relative;
    display: inline-block;
    width: 100vw;
    height: 50vh;
    
    //Text
    text-align: center;
    line-height: 2rem;
    
    label {
      //Layout
      position: relative;
      width: 100vw;
      
      //Text
      font-size: 30px;
      font-weight: bold;
    }
    
    input {
      //Layout
      position: relative;
      width: 80%;
      height: 8vh;
      min-height: 40px;
      padding: 10px;
      
      //Text
      text-align: center;
      
      //Design
      border-radius: 20px;
      border: 0;
      outline: none;
      box-shadow: 0 0 5px black;
    }
  }
}

#bottom-half {
  //Layout
  position: relative;
  width: 100vw;
  height: 50vh;
  min-height: 200px;
  border: 1px solid;
  
  //Text
  font-size: 20px;
  line-height: 50vh;
  text-align: center;
  
  //Design
  color: $primary-dark;
  
  p {
    //Layout
    position: relative;
    display: inline-block;
    
    //Text
    line-height: 1.5rem;
  }
}
!

JS 

              
                const arabicEntry = document.querySelector("#arabic-entry");
const output      = document.querySelector("#output");

function convertToRoman(input, output) {
  //Set initial variables
  let romanArr = [];
  let num = input.value;
  
  //Check to ensure input consists solely of digits
  let numCheck = /\d/;
  num = num
    .toLowerCase()
    .split('')
    .filter(char => numCheck.test(char)).
    join('');
  
  //Ensure input field does not display invalid characters
  input.value = num;
  
  //If number is too great, alert user and bail
  if (num > 3999) {
    output.innerHTML = "Let's stick to values <4000. The <em>apostrophus</em> or <em>vinculum</em> methods are beyond the scope of this simple trinket.";
    return;
  }
  
  //Convert user's number into array that represents in the input's expanded form
  let convertNum = num  //E.g., 1004
    .toString()         //E.g., "1004"
    .split('')          //E.g., ["1", "0", "0", "4"]
    .reverse()          //E.g., ["4", "0", "0", "1"]
    .map((char, ind) => {
      return char * Math.pow(10,ind);
    })                  //E.g., [4, 0, 0, 1000]
    .reverse();         //E.g., [1000, 0, 0, 4] => Ready for conversion to Roman numerals
  
  //Simple object pairing Arabic and Roman numerals
  let numsObj = {
    "0": "",
    "1": "I",
    "2": "II",
    "3": "III",
    "4": "IV",
    "5": "V",
    "6": "VI",
    "7": "VII",
    "8": "VIII",
    "9": "IX",
    "10": "X",
    "20": "XX",
    "30": "XXX",
    "40": "XL",
    "50": "L",
    "60": "LX",
    "70": "LXX",
    "80": "LXXX",
    "90": "XC",
    "100": "C",
    "200": "CC",
    "300": "CCC",
    "400": "CD",
    "500": "D",
    "600": "DC",
    "700": "DCC",
    "800": "DCCC",
    "900": "CM",
    "1000": "M",
    "2000": "MM",
    "3000": "MMM"
  };
  
  //Convert each numeral in array to appropriate Roman counterpart
  for (let i = convertNum.length - 1; i >= 0; i--) {
    romanArr.unshift(numsObj[convertNum[i]]);
  }
  
  //Display output
  output.innerHTML = num !== "" ? romanArr.join('') : "Nothing's been entered...";
}

arabicEntry.addEventListener("input", () => { convertToRoman(arabicEntry, output); });
!
999px

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