My solution for the challenge!

// Run script once content is loaded
document.addEventListener('DOMContentLoaded', () => {
  // Target DOM elements
  const form = document.getElementById('registrar');
  const input = form.querySelector('input');

  const mainDiv = document.querySelector('.main');
  const ul = document.getElementById('invitedList');

  const div = document.createElement('div');
  const filterLabel = document.createElement('label');
  const filterCheckBox = document.createElement('input');

  // Add filter checkbox to the page
  filterLabel.textContent = "Hide those who haven't responded";
  filterCheckBox.type = 'checkbox';
  div.appendChild(filterLabel);
  div.appendChild(filterCheckBox);
  mainDiv.insertBefore(div, ul);
  // Filter out unconfirmed guests when checked
  filterCheckBox.addEventListener('change', (e) => {
    const isChecked = e.target.checked;
    const lis = ul.children;
    if(isChecked) {
      for (let i = 0; i < lis.length; i += 1) {
        let li = lis[i];
        let label = li.children[1];
        if (li.className === 'responded') {
          li.style.display = '';  
          // Hide 'confirm' label and checkbox on guest cards when filter is active
          label.style.display = 'none';
        } else {
          li.style.display = 'none';    
        }
      }
    } else {
      // Remove filter when unchecked
      for (let i = 0; i < lis.length; i += 1) {
        let li = lis[i];
        let label = li.children[1];
        li.style.display = '';
        label.style.display = '';
      }                                 
    }
  });

  // Check browser support for local storage
  if (typeof(Storage) !== "undefined")  {

    function createLI(text) {
      function createElement(elementName, property, value) {
        const element = document.createElement(elementName);  
        element[property] = value; 
        return element;
      }

      function appendToLI(elementName, property, value) {
        const element = createElement(elementName, property, value);     
        li.appendChild(element); 
        return element;
      }

      const li = document.createElement('li');
      appendToLI('span', 'textContent', text);     
      appendToLI('label', 'textContent', 'Confirm')
        .appendChild(createElement('input', 'type', 'checkbox'));
      appendToLI('button', 'textContent', 'edit');
      appendToLI('button', 'textContent', 'remove');
      return li;
    }

     // Retrieve guest list from local storage
     function getCurrentGuests() {
      const currentGuests = localStorage.getItem('guests');
      if (currentGuests) {
        return JSON.parse(currentGuests);
      } else {
        return [];
      }
    }

    // Retrieve all entries on page load
    const guestList = getCurrentGuests();

    // Add existing guests to ul
    let retrieveGuestList = '';
    getCurrentGuests().forEach( (guest) => {
      retrieveGuestList += guest;
    });
    ul.innerHTML = retrieveGuestList;

    // Add guest to local storage
    function addGuest(guestList) {
      localStorage.setItem('guests', JSON.stringify(guestList));
    }

    // Remove guest from local storage
    function removeGuest(name) {
      guestList.forEach( (guest) => {
        if (guest.includes(name)) {
          const removedGuest = guest;
          const index = guestList.indexOf(removedGuest);
          guestList.splice(index, 1);
          addGuest(guestList);
        }
      });
    }

    form.addEventListener('submit', (e) => {
      e.preventDefault();
      const text = input.value;
      // Register existing guests in a variable to check for duplicates
      const addedGuests = document.querySelectorAll('li span');
      let existingNames = [];
      addedGuests.forEach((name) => {
        existingNames += name.textContent;
        return existingNames;
      });
      // Empty input box after submitting guest
      input.value = '';
      // Check if input is blank
      if (text === '') {
        alert(`Sorry! We didn't get that name, could you please try again?`);
        // Check if input is a duplicate
      } else if (existingNames.includes(text)) {
          alert(`${text} is already in your guestlist!`);
        // Prevent new guests entries from showing when filter is active  
      } else if (filterCheckBox.checked) {
      const li = createLI(text);
      ul.appendChild(li);
      li.style.display = 'none';
        // Add new guest entry to guest list
      } else {
      const li = createLI(text);
      ul.appendChild(li);  
      // Add guests to local storage
      guestList.push(li.outerHTML);
      addGuest(guestList);
      }
    });

    ul.addEventListener('change', (e) => {
      const checkbox = event.target;
      const checked = checkbox.checked;
      const listItem = checkbox.parentNode.parentNode;
      const labelText = checkbox.parentNode.childNodes[0];
      if (checked) {
        listItem.className = 'responded';
        labelText.textContent = 'Confirmed';
      } else {
        listItem.className = '';
        labelText.textContent = 'Confirm';
      }
    });

    ul.addEventListener('click', (e) => {
      if (e.target.tagName === 'BUTTON') {
        const button = e.target;
        const li = button.parentNode;
        const ul = li.parentNode;
        const span = li.firstElementChild;
        let guestName = document.querySelector('li span');
        const action = button.textContent;
        const nameActions = {
          remove: () => {
            ul.removeChild(li);
            guestName = span.textContent;
            removeGuest(guestName);
          },
          edit: () => {
            const span = li.firstElementChild;
            const input = document.createElement('input');
            input.type = 'text';
            input.value = span.textContent;
            li.insertBefore(input, span);
            li.removeChild(span);
            button.textContent = 'save';  
          },
          save: () => {
            const input = li.firstElementChild;
            const span = document.createElement('span');
            span.textContent = input.value;
            li.insertBefore(span, input);
            li.removeChild(input);
            button.textContent = 'edit';        
          }
        };

        // select and run action in button's name
        nameActions[action]();
      }
    }); 
  } 
});