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Create a Hex to RGB Convertor App with HTML, CSS, and JavaScript (1/2)

December 31, 2019

With just a little knowledge of HTML, CSS, and JavaScript (JS), you can create something really great. This tutorial aims to showcase how you can combine your knowledge of these three languages and build a functional app.

What are we building?

As you may have read in the title, the app you’ll build at the end of this lesson takes a hex (short for hexadecimal) color (e.g #457a7a) as input and returns an rgb equivalent as output (rgb(69,122,122)).

Let call our app RGBizer App. As a bonus feature, the submitted color will be the background color on the right side of the page, and as the color for the RGB text.

You can see the finished app


We will be building our app in two parts. This is the first one, and it is focusing on HTML and CSS.

The Basic Formula

It all starts with the HTML.

    <div class="main-child">
      <h1>Color Hex to RGB convertor</h1>
      <p>Enter the hexadecimal code of your favarite color in the box below to get rgb() equivalente</p>
          <label for="hex-code">#</label>
          <input type="text" placeholder="Enter hex code without #" name="hex-code" id="hex-code">
          <input type="submit" value="Convert">
      <div id="wrapper-rgb">
        <p>RGB color is <span id="rgb"></span></p>
    <div id="preview" class="main-child">
        <p>This is your color preview</p>

We create a simple form with an input where the user can enter the hex code, and a submit button to start conversion. A hex color always starts with #, so no need to enter it, and has six symbols in the range [0-9a-f].

The output will be printed in the span element above after the user has submitted the form.

The Style with CSS Flex

Let’s start by the body:

body {
    box-sizing: border-box;
    margin: 0;

We use box-sizing: border-box because it makes it much easier to size elements. margin: 0 let us occupy the whole viewpoint.

Next, we style main element as a flex container with a height of 45em as this…

main {
      display: flex;    
      height: 47em;                                

…so that we have two flex items placed in the horizontal axis (row).

In order to let us easily center content horizontally and vertically, we make our two flex items flex containers:

.main-child {
  display: flex; 
  flex-direction: column;
  width: 50%;
  height: 100%;
  justify-content: center;
  align-items: center;

The content is aligned in the column (flex-direction: column) and occupy all available space in height and half of it in width. To center horizontally we use justify-content: center and align-items: center is used to center vertically. We can add background: green to the second bloc just to distinguish our two blocs, and have in mind that we will be applying background color there every time the user submit a color.

.main-child:last-child {
  background: green;

For the text in the .main-child:last-child, let’s use floralwhite for the color:

.main-child:last-child p {
    color: floralwhite;

To finish, let’s add some style to the :first-child of .main-child class.

.main-child:first-child input[type="text"] {
    width: 25em;

.output {
  visibility: hidden;

#rgb {
    font-weight: bold;

Nothing tricky here. We have modified a bit our markup to add .output class to the div element so that we hide it.


In this first part of our tutorial, we have defined the markup of our app (do you remember it name? RGBizer). To style it we used CSS flexbox. In the second and last part of this tutorial, you will learn how to make it functional with JavaScript. You can see this state of our app here


Before you go, thank you for reading. If you found this interesting don’t hesitate to share with friends.

Happy hacking!