This is the seventh post in a series examining modern CSS solutions to problems I've been solving over the last 13+ years of being a frontend developer. Visit ModernCSS.dev to view the whole series and additional resources.

This technique explores using:

• Animation with CSS transition and transform
• Using the :focus-within pseudo-class
• CSS grid for positioning
• dynamic centering technique
• Accessibility considerations for dropdown menus

If you've ever pulled your hair out dealing with the concept of "hover intent", then this upgrade is for you!

Before we get too far, while our technique 100% uses only CSS, there is a need to add some Javascript for a more comprehensively accessible experience. There is also a polyfill needed for a key feature to make this work - :focus-within - for the most reliable support. But we've still greatly improved from the days of needing one or more jQuery plugins to accomplish the visual effects.

Accessibility update - 08/18/20: A huge thanks to Michael Fairchild of Deque (and creator of the excellent resource a11ysupport.io) for testing the original solution across various assistive technology. The CSS-only method needs some Javascript to fully meet WCAG 2.1. In particular, javascript needs to be used to offer a non-mouse/non-tab way to dismiss the menu (think escape key) to meet success criteria 1.4.13. Michael pointed to this WAI-ARIA Authoring Practices demo which provides more info on the necessary Javascript features. These are highly recommended additions for your final production solution.

If you've not used Sass, you may want to take five minutes to understand the nesting syntax of Sass to most easily understand the code samples given.

Base Navigation HTML

We will enhance this as we continue, but here's our starting structure:

<nav aria-label="Main Navigation">
  <ul>
    <li><a href="#">About</a></li>
    <li class="dropdown">
    <!-- aria-expanded needs managed with Javascript -->
      <button type="button" class="dropdown__title" aria-expanded="false" aria-controls="sweets-dropdown">
        Sweets
      </button>
      <ul class="dropdown__menu" id="sweets-dropdown">
        <li><a href="#">Donuts</a></li>
        <li><a href="#">Cupcakes</a></li>
        <li><a href="#">Chocolate</a></li>
        <li><a href="#">Bonbons</a></li>
      </ul>
    </li>
    <li><a href="#">Order</a></li>
  </ul>
</nav>

Overlooking the button for a minute, this is the semantic standard for navigation links. This structure is flexible to live anywhere on your page, so it could be a table of contents in your sidebar as easily as it is the main navigation.

Right out the gate, we have implemented a few features specifically for accessibility:

1. aria-label on the <nav> to help identify it's purpose when assistive tech is used to navigate a page by landmarks
2. Use of a button as a focusable, discoverable element to trigger the opening of the dropdown
3. aria-controls on the .dropdown__title that links to the id of the .dropdown__menu to associate it with the menu for assistive tech
4. aria-expanded on the button which in your final solution needs toggled via Javascript as noted in the demo mentioned at the beginning of this article

As noted by Michael, use of a button element also allows Dragon Naturally Speaking users to say something like 'click button' to open the menu.

Our (mostly) default starting appearance is as follows:

Initial Navigation Styles

First, we'll give some container styles to nav and define it as a grid container. Then we'll remove default list styles from the nav ul and nav ul li.

nav {
  background-color: #eee;
  padding: 0 1rem;
  display: grid;
  place-items: center;

  ul {
    list-style: none;
    margin: 0;
    padding: 0;
    display: grid;

    li {
      padding: 0;
    }
  }

}

We've lost the hierarchical definition, but we can begin to bring it back with the following:

nav {
  // ...existing styles

  > ul {
    grid-auto-flow: column;

    > li {
      margin: 0 0.5rem;
    }
  }
}

By using the child combinator selector > we've defined that the top-level ul which is a direct child of nav should switch it's grid-auto-flow to column which effectively updates it to be along the x-axis. We then add margin to the top-level li elements for a bit more definition. Now, the future dropdown items are appearing contained below the "Sweets" menu and are more clearly its children:

Next we'll add a touch of style first to all links as well as the .dropdown__title, then to only the top-level links in addition to the .dropdown__title. This is also where we clear out the native browser styles inherited for button elements.

// Clear native browser button styles
.dropdown__title {
  background-color: transparent;
  border: none;
  font-family: inherit;
}

nav {
  > ul {

    > li {
      // All links contained in the li
      a,
      .dropdown__title {
        text-decoration: none;
        text-align: center;
        display: inline-block;
        color: blue;
        font-size: 1.125rem;
      }

      // Only direct links contained in the li
      > a,
      .dropdown__title {
        padding: 1rem 0.5rem;
      }

    }
  }
}

Base Dropdown Styles

We have thus far been relying on element selectors, but we will bring in class selectors for the dropdown since there may be multiple in a given navigation list.

We'll first style up the .dropdown__menu and its links to help identify it more clearly as we work through positioning and animation:

.dropdown {
  position: relative;

  .dropdown__menu {
    background-color: #fff;
    border-radius: 4px;
    box-shadow: 0 0.15em 0.25em rgba(black, 0.25);
    padding: 0.5em 0;
    min-width: 15ch;

    a {
      color: #444;
      display: block;
      padding: 0.5em;
    }
  }
}

One of the clear issues is that the .dropdown__menu is affecting the nav container, which you can see from the grey nav background being present around the dropdown.

We can start to fix this by adding position: absolute to the .dropdown__menu which takes it out of normal document flow:

You can see it's aligned to the left and below of the parent list item. Depending on your design, this may be the desirable location.

We're going to pull out a centering trick to align the menu central to the list item:

.dropdown__menu {
  // ... existing styles

  position: absolute;

  // Pull up to overlap the parent list item very slightly
  top: calc(100% - 0.25rem);
  // Use the left from absolute position to shift the left side
  left: 50%;
  // Use translateX to shift the menu 50% of it's width back to the left 
  transform: translateX(-50%);
}

The magic of this centering technique is that the menu could be any width or even a dynamic width and it would center appropriately.

Dropdown Reveal Styles

There are two primary triggers we want used to open the menu: :hover and :focus.

However, traditional :focus will not persist the open state of the dropdown. Once the initial trigger loses focus, the keyboard focus may still move through the dropdown menu, but visually the menu would disappear.

:focus-within

There is an upcoming pseudo-class called :focus-within and it is precisely what we need to make it possible for this to be a CSS-only dropdown. As mentioned in the intro, it does require a polyfill if you need to support IE < Edge 79 (you do... for now).

From MDN, italics mine to show the part we're going to benefit from:

The :focus-within CSS pseudo-class represents an element that has received focus or contains an element that has received focus. In other words, it represents an element that is itself matched by the :focus pseudo-class or has a descendant that is matched by :focus.

Hide the dropdown by default

Before we can reveal the dropdown, we need to hide it, so we will use the hidden styles as the default state.

Your first instinct may be display: none but that locks us out of gracefully animating the transition.

Next, you might try simply opacity: 0 which visibly hides it but leaves behind "ghost links" because the element still has computed height.

Instead, we will use a combination of opacity, transform, and visibilty:

.dropdown__menu {
  // ... existing styles
  transform: rotateX(-90deg) translateX(-50%);
  transform-origin: top center;
  opacity: 0.3;
}

We add opacity but not all the way to 0 to enable a bit smoother effect later.

And, we update our transform property to include rotateX(-90deg), which will rotate the menu in 3D space to 90 degrees "backwards". This effectively removes the height and will make for an interesting transition on reveal. Also you'll notice the transform-origin property which we add to update the point around which the transform is applied, versus the default of the horizontal and vertical center.

Additionally, to meet success criteria 1.3.2, the links should be hidden from screen reader users until they are visually displayed. We ensure this behavior by including visibility: hidden (thanks again to Michael for this tip!).

Before we do the reveal, we need to add a transition property. We add it to the main .dropdown__menu rule so that it applies both on and off focus/hover, aka "forwards" and "backwards".

.dropdown__menu {
  // ... existing styles
  transition: 280ms all ease-out;
}

Revealing the dropdown

With all that prior setup, revealing the dropdown on both hover and focus can be accomplished as succinctly as:

.dropdown {
  // ... existing styles

  &:hover,
  &:focus-within {

    .dropdown__menu {
      opacity: 1;
      transform: rotateX(0) translateX(-50%);
      visibility: visible;
    } 

  }

}

First, we reverse the visibilty (or the other properties would not work), and then we've reversed the rotateX by resetting to 0, and then bring the opacity all the way up to 1 for full visibility.

Here's the result:

The rotateX property allows the appearance of the menu swinging in from the back, and opacity just makes it a little softer transition overall.

Once again a note that for full accessibility, there is a need for Javascript to fully handle for keyboard assistive tech events that do not always trigger :focus. This means some sighted keyboard users may discover the dropdown links, but without a :focus event emitted, they will not see the dropdown menu actually open. Review the w3c demo for how to finish incorporating Javascript in this solution.

Handling Hover Intent

If you've been at this web thing for a while, I'm hoping the following will make you go 🤯

When I first began battling dropdown menus I was creating them primarily for IE7. On a big project, several team members asked something along the lines of "can you stop the menu appearing if I'm just scrolling/mousing over the menu?". The solution I finally found after much Googling (including trying to come up with the right phrase to get what I was after) was the hoverIntent jQuery plugin.

I needed to set that up because since we are using the transition property, we can also add a very slight delay. For general purposes, this will prevent the dropdown animation triggering for "drive-by" mouseovers.

Order matters when we're defining all transition properties in one line, and the second numerical value in order will be picked up as the delay value:

.dropdown__menu {
  // ... existing styles
  transition: 280ms all 120ms ease-out;
}

Check out the results:

It takes a pretty leisurely rollover to trigger the menu, which we can loosely infer as intent to open the menu. The delay is still short enough to not be consciously noticed prior to opening the menu, so it's a win!

You may still choose to use Javascript to enhance this particularly if it's going to launch a "mega menu" that would be more disruptive, but this is still pretty delightful.

Dropdown Menu Indicator

Hover intent is one thing, but really we need an additional cue to the user that this menu has additional options. An extremely common convention is a "caret" or "down arrow" mimicking the indicator of a native select element.

To add this, we will update the .dropdown__title styles. We'll define it as an inline-flex container and then create an :after element that uses the border trick to create a downward arrow. We use a dash of translateY() to optically align it with our text:

.dropdown {
  // ... existing styles

    .dropdown__title {
      display: inline-flex;
      align-items: center;

      &:after {
        content: "";
        border: 0.35rem solid transparent;
        border-top-color: rgba(blue, 0.45);
        margin-left: 0.25em;
        transform: translateY(0.15em);
      }
    }

}

Closing the menu on mobile

Here's another place where ultimately you may have to enhance with Javascript.

To keep it CSS-only, and acceptable for non-application websites, you need to apply tabindex="-1" on the body, effectively allowing any clicks outside of the menu to remove focus from it and allowing it to close.

This is a bit of a stretch - and it may be a little frustrating to users - so you may want to enhance this to hide on scroll as well with Javascript especially if you define the nav to use position: sticky and scroll with the user.

Final Result

Here's the final result with a bit of extra styling including an arrow to more visually connect the menu to the link item, custom focus states on all the nav links, and position: sticky on the nav: