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HTML

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CSS

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CSS Base

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="modal" role="dialog" aria-labelledby="Modal_Title" aria-describedby="Modal_Description" aria-hidden="true" style="display: none">
    <div class="modal-content">
        <h2 id="Modal_Title">Delete Confirmation</h2>
        <p id="Modal_Description" class="visually-hidden">Some visually hidden text describing how to use the modal.</p>
        <input type="text" value="first focusable element"><br><br>
        <button class="close-modal">Close Modal</button>
    </div>
</div>
<main>
    <button class="open-modal">Open Modal</button>
    
    <br>
    <br>
    <br>
    <button>This element shouldnt receive focus when the modal is open</button>
</main>
!

CSS 

              
                .modal {
    position: fixed;
    background: rgba(0, 0, 0, .6);
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    padding: 20px;
    display: flex;
    align-items: center;
    justify-content: center;
    flex-direction: column;
}

.modal-content {
    width: 250px;
    height: 250px;
    background: white;
    padding: 20px;
}

.visually-hidden {
  position: absolute !important;
  height: 1px; width: 1px; 
  overflow: hidden;
  clip: rect(1px, 1px, 1px, 1px);
}

*:focus {
    outline: 3px dashed blue;
}
!

JS 

              
                // Open this pen in debug mode for testing
// https://codepen.io/noahblon/debug/yJpXZa
// This is a sketch of some principles of an accessible modal - trying to keep it simple.  For a more complete demonstration, check out: https://github.com/gdkraus/accessible-modal-dialog

const FOCUSABLE_SELECTORS = 'a[href], area[href], input:not([disabled]), select:not([disabled]), textarea:not([disabled]), button:not([disabled]), iframe, object, embed, *[tabindex], *[contenteditable]';

const openModalBtn = document.querySelector('.open-modal');
const closeModalBtn = document.querySelector('.close-modal');
const modal = document.querySelector('.modal');
const main = document.querySelector('main');

function openModal() {
    // show the modal
    modal.style.display = 'flex';
    
    // Focus the first element within the modal. Make sure the element is visible and doesnt have focus disabled (tabindex=-1);
    modal.querySelector(FOCUSABLE_SELECTORS).focus();

    // Trap the tab focus by disable tabbing on all elements outside of your modal.  Because the modal is a sibling of main, this is easier. Make sure to check if the element is visible, or already has a tabindex so you can restore it when you untrap.    
    const focusableElements = main.querySelectorAll(FOCUSABLE_SELECTORS);
    focusableElements.forEach(el => el.setAttribute('tabindex', '-1'));

    // Trap the screen reader focus as well with aria roles. This is much easier as our main and modal elements are siblings, otherwise you'd have to set aria-hidden on every screen reader focusable element not in the modal.
    modal.removeAttribute('aria-hidden');
    main.setAttribute('aria-hidden', 'true');
}

function closeModal() {
    // hide the modal
    modal.style.display = 'none';

    // Untrap the tab focus by removing tabindex=-1. You should restore previous values if an element had them.    
    const focusableElements = main.querySelectorAll(FOCUSABLE_SELECTORS);
    focusableElements.forEach(el => el.removeAttribute('tabindex'));
    
    // Untrap screen reader focus
    modal.setAttribute('aria-hidden', 'true');
    main.removeAttribute('aria-hidden');
    
    // restore focus to the triggering element
    openModalBtn.focus();
}

openModalBtn.addEventListener('click', openModal);
closeModalBtn.addEventListener('click', closeModal);
!
999px

Console