This is the nineteenth 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.

We'll create custom, cross-browser, theme-able, scalable checkboxes in pure CSS with the following:

• currentColor for theme-ability, including of the SVG
• em units for relative sizing
• use of SVG for the :checked indicator
• CSS grid layout to align the input and label

Many of the concepts here overlap with our custom styled radio buttons from episode 18, with the addition of using an SVG for the :checked state and including styling for the :disabled state

Now available: my egghead video course Accessible Cross-Browser CSS Form Styling. You'll learn to take the techniques described in this tutorial to the next level by creating a themable form design system to extend across your projects.

Checkbox HTML

In the radio buttons article, we explored the two valid ways to markup input fields. Much like then, we will select the method where the label wraps the input.

For our checkboxes, we're going to make use of an inline SVG as well for our custom control, so here's our base HTML for testing both an unchecked and checked state:

<label class="checkbox">
  <span class="checkbox__input">
    <input type="checkbox" name="checkbox">
    <span class="checkbox__control">
      <svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' aria-hidden="true" focusable="false">
        <path fill='none' stroke='currentColor' stroke-width='3' d='M1.73 12.91l6.37 6.37L22.79 4.59' /></svg>
    </span>
  </span>
  <span class="radio__label">Checkbox</span>
</label>

<label class="checkbox">
  <span class="checkbox__input">
    <input type="checkbox" name="checked" checked>
    <span class="checkbox__control">
      <svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 24 24' aria-hidden="true" focusable="false">
        <path fill='none' stroke='currentColor' stroke-width='3' d='M1.73 12.91l6.37 6.37L22.79 4.59' /></svg>
    </span>
  </span>
  <span class="radio__label">Checkbox - Checked</span>
</label>

Note the use of aria-hidden="true" and focusable="false", since we're going to treat this SVG like a decorative icon. In this case, the underlying checkbox is still available to provide the semantics of the checked state for accessibility.

Common Issues With Native Checkboxes

As with the radio buttons, the checkbox appearance varies across browsers.

Here's the base styles across (in order from left) Chrome, Firefox, and Safari:

Also like with radio buttons, the checkbox doesn't scale along with the font-size.

Our solution will accomplish the following goals:

• scale with the font-size provided to the label
• gain the same color as provided to the label for ease of theme-ability
• achieve a consistent, cross-browser design style, including :focus state
• maintain keyboard accessibility

Our styles will begin with the same variable and reset as used for the radio buttons

Label Styles

Our label uses the class of .checkbox. The base styles we'll include here are the font-size and color. Recall from earlier that the font-size will not yet have an effect on the visual size of the checkbox input.

We're using an abnormally large font-size just to emphasize the visual changes for purposes of the tutorial demo.

Our label is also the layout container for our design, and we're going to set it up to use CSS grid layout to take advantage of grid-gap.

.checkbox {
    display: grid;
    grid-template-columns: min-content auto;
    grid-gap: 0.5em;
    font-size: 2rem;
    color: var(--color);
}

Here's our progress as captured in Chrome, with Inspector revealing grid lines:

Since we defined the stroke color of our SVG to be currentColor it has picked up the rebeccapurple value as well.

Custom Checkbox Style

To prepare for this, we have wrapped our input in a span with the class checkbox__input. Then, we have also added a span as a sibling of the input with the class checkbox__control. The control span also contains the checkmark SVG.

Order here matters as we'll see when we style for :checked, :focus, and :disabled.

Step 1: Hide the Native Checkbox

We need to hide the native checkbox, but keep it technically accessible to enable proper keyboard interaction and also to maintain access to the :checked, :focus state.

To accomplish this, we'll use opacity to visually hide it, and set its width and height to 1em. We will retain width and height to ensure discoverability by users of touch devices.

.checkbox__input {

  input {
    opacity: 0;
    width: 1em;
    height: 1em;
  }

}

I highly recommend this article from Sara Soueidan which goes in-depth on the options for inclusively hiding both radio buttons and checkboxes, and also offers a slightly different take on custom styling of these inputs.

Step 2: Custom Unchecked Checkbox Styles

For our custom checkbox, we'll attach styles to the span.checkbox__control that is the sibling following the input.

We'll define it as an inline-grid element that is sized using em to keep it relative to the font-size applied to the label. We also use em for the border-width value to maintain the relative appearance. We then use border-radius to softly round the corners.

.checkbox__control {
  display: inline-grid;
  width: 1em;
  height: 1em;
  border-radius: 0.25em;
  border: 0.1em solid currentColor;
}

Here's our progress after hiding the native input and defining these base styles for the custom checkbox control:

Well, the display: inline-grid performed its magic to nicely contain the SVG checkmark, but what the deal with the alignment of the custom control vs. the label?

Since we retained a width and height on the checkbox input and only hide it with opacity, it's still taking up space.

To resolve this, we'll use CSS grid layout to define the .checkbox__input as a single grid template area, and direct its children to all occupy that area. This technique is the modern replacement for position: absolute.

.checkbox__input {
  display: grid;
  grid-template-areas: "checkbox";

  > * {
    grid-area: checkbox;
  }
}

If you find that you have a bit of vertical misalignment between the control and label due to the font in use, see the related section for the radio buttons for a solution using transform

Step 3: Styling :checked vs Unchecked State

Our use of opacity: 0 has kept the native checkbox input accessible for keyboard interaction as well as click/tap interaction.

It has also maintained the ability to detect its :checked state with CSS.

Remember how I mentioned order matters? Thanks to our custom control following the native input, we can use the adjacent sibling combination - + - to style our custom control when the native control is :checked 🙌

However - since we are using an inline SVG to display the :checked indicator, we actually need to set it up so that the SVG is initially hidden and only shown when in the :checked state.

We'll add a transition to provide an animated effect upon switching between states, and use transform: scale(0) to do the initial hiding:

.checkbox__control svg {
  transition: transform 0.1s ease-in 25ms;
  transform: scale(0);
  transform-origin: bottom left;
}

Next, we need to add the :checked styles to scale it back up into view:

.checkbox__input input:checked
 + .checkbox__control svg {
  transform: scale(1);
}

Here's a demo of the animated :checked interaction:

Step 4: The :focus state

For the :focus state, we're going to use the same double box-shadow technique as we used for the radio buttons. This lets us leverage currentColor but ensure distinction between the base custom checkbox and the :focus style.

.checkbox__input input:focus
 + .checkbox__control {
  box-shadow: 0 0 0 0.05em #fff, 0 0 0.15em 0.1em currentColor;
}

The order of box-shadow definitions means we are first creating the appearance of a thin white border, which appears above a feathered out shadow that takes on the value from currentColor.

Here's a demo of the :focus appearance:

Step 5: Styles For :disabled Checkboxes

One step we missed in the radio buttons tutorial was styling for the :disabled state.

This will follow a similar pattern as for our previous states, with the change here mostly being to update the color to a grey. Since all of our colors are tied to the currentColor value, this is an easy update for the custom control:

:root {
  --disabled: #959495;
}

.checkbox__input input:checkbox:disabled
 + .checkbox__control {
  color: var(--disabled);
}

But we've hit a snag. Since the label is the parent element, we don't currently have a way in CSS alone to style it based on the :disabled state.

For a CSS-only solution, we need to create an add an extra class to the label when it is known that the checkbox is disabled. Since this state can't be changed by the user, this will generally be an acceptable additional step.

We'll create the class of .checkbox--disabled to be added to the HTML label element.

.checkbox--disabled {
  color: var(--disabled);
}

Demo

Here's a demo that includes the :disabled styles, and also shows how the power of CSS variables + the use of currentColor means we can re-theme an individual checkbox with a simple inline style. This is very useful for things like a quick change to an error state.