Hey there! The issue with using display: none/block to toggle your modal is that CSS transitions don’t work on the display property — it’s an instant, all-or-nothing switch. Let’s fix this with CSS transitions for smooth fade effects and a tiny tweak to your JavaScript to handle state changes properly.

Step 1: Update Modal CSS #

First, replace your base .modal styles to use opacity and visibility (properties that support transitions) instead of display. We’ll also add a helper class to toggle the modal’s visible state:

/* The Modal (background) */
.modal {
  /* Remove display: none */
  opacity: 0; /* Fully transparent by default */
  visibility: hidden; /* Hides from screen readers & layout interactions */
  pointer-events: none; /* Prevents clicks on hidden modal */
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.9);
  padding-top: 50px;
  /* Add smooth transition for fade effect */
  transition: opacity 0.3s ease, visibility 0.3s ease;
}

/* Active state to show modal */
.modal.active {
  opacity: 1;
  visibility: visible;
  pointer-events: auto; /* Re-enable interactions when visible */
}

Keep all your existing modal styles (like .modal-content, .containerlog, etc.) — we’re only modifying the base .modal class and adding the .active toggle class.

Step 2: Update HTML Onclick Handlers #

Instead of directly setting style.display, we’ll toggle the .active class to show/hide the modal:

<!-- Updated login button -->
<a href="#" onclick="document.getElementById('id01').classList.add('active')">Prihlásiť sa</a>

<!-- Updated close button -->
<span onclick="document.getElementById('id01').classList.remove('active')" class="close" title="Close Modal">&times;</span>

Step 3: Fix "Click Outside to Close" JavaScript #

Update your existing script to remove the .active class instead of changing display:

// Get the modal
var modal = document.getElementById('id01');

// Close modal when clicking outside of it
window.onclick = function(event) {
  if (event.target == modal) {
    modal.classList.remove('active');
  }
}

Bonus: Smooth Transition for Login/Register Form Toggle #

If you want the login/register forms to also transition smoothly, apply the same logic to their container classes:

.containerreg, .containerlog {
  opacity: 0;
  visibility: hidden;
  pointer-events: none;
  transition: opacity 0.3s ease, visibility 0.3s ease;
}

.containerreg.active, .containerlog.active {
  opacity: 1;
  visibility: visible;
  pointer-events: auto;
}

Then update their onclick handlers to toggle the .active class, and add the .active class to your default visible form (.containerlog):

<!-- Default visible login form -->
<div class="containerlog active" id="containerlog">
<!-- ... rest of login form ... -->
<a href="#" onclick="document.getElementById('containerreg').classList.add('active'); document.getElementById('containerlog').classList.remove('active')" class="accounthave">Nemáte účet? <b>Registrujte sa.</b></a>

<!-- Register form -->
<div class="containerreg" id="containerreg">
<!-- ... rest of register form ... -->
<a href="#" onclick="document.getElementById('containerreg').classList.remove('active'); document.getElementById('containerlog').classList.add('active')" class="accounthave">Máte už účet? <b>Prihláste sa.</b></a>

Why This Works #

• opacity controls transparency and supports smooth transitions.
• visibility ensures hidden elements are ignored by screen readers and layout logic (unlike opacity:0 which still occupies space).
• pointer-events: none prevents accidental clicks on hidden elements.
• Toggling the .active class triggers the CSS transition, creating a natural fade-in/out effect.

内容的提问来源于stack exchange，提问作者user9247305