As I hinted to in the previous installment of this series; a radio button is in the same situation as checkboxes on the web: no options for styling the native component, but the same strategy we used for checkboxes can be applied to radio buttons as well.

The goals

Style the radio button
Style the label
Require no additional DOM nodes or configuration (including icons/images)
Require 0 JavaScript

The results

https://codepen.io/kallmanation/pen/BajyWmp

The strategy

Hide the actual radio input
Show a styled element that looks like an empty radio button when the input is unchecked
Show a styled element that looks like a selected radio button when the input is checked

How to get there

The CSS selectors used

Type selector type - selects all elements of the given type (e.g. input will select all <input ... /> nodes)
Attribute selector [attribute="value"] - selects an element with attribute where its value is exactly value
Psuedo-class :checked - selects checkbox/radio input types or options in a select that are selected/checked/on/active
Psuedo-element ::before - styleable element that doesn't actually exist in the DOM; considered the first child of the selected element
Universal selector * - selects anything/everything
Child combinator > - combines two selectors; narrowing the selection on the right-hand side to only those elements that are direct descendants of elements selected by the left-hand side.
Adjacent sibling combinator + - combines two selectors; narrowing the selection on the right-hand side to only those elements that are the siblings immediately after the elements on the left-hand side

Important CSS styles used

content - used in the ::before psuedo-element to set its content
display - specifically none to hide elements and inline-block to make our otherwise inline radio button able to have a consistent width and height
width / height - does what you think: sets width and height of the element
color - sets the color of the text
text-align / vertical-align - used for adjusting the position of our radio button to its label
border styles - How we'll form and color the radio button
radial-gradient - used in background to fill in the radio button in the classic "half-full" style

Starting out: the HTML

Let's set up our radio button as a child of its label element with a sibling of a span of the label text:

<label>
  <input type="radio" name="key" value="value" checked />
  <span>I am a radio button</span>
</label>
<label>
  <input type="radio" name="key" value="another-value" />
  <span>I am another radio button</span>
</label>

This structure allows clicking on the label text to select the radio without needing for or unique id attributes (Note it's important the name is the same for at least 2 or more radio buttons. This creates the "radio group".). Placing the text in a span directly after the input will allow us to select it in CSS.

First step: hide the unstyleable radio

Going back to our strategy: since we can't do anything with the native radio button, we'll have to hide it and do our own thing.

label > input[type="radio"] {
  display: none;
}

We'll select the radio button (input[type="radio"]) and make sure it's labelled the way we need it to be (label >). Then just display: none to get it off our screens.

Second step: make our own radio button

Making an empty circle is easy with CSS, just put a border around a square element and give it a border radius of 50%. As with the checkbox, we'll put this on the ::before psuedo-element to avoid extra markup.

label > input[type="radio"] + *::before {
  content: "";
  display: inline-block;
  vertical-align: bottom;
  width: 1rem;
  height: 1rem;
  margin-right: 0.3rem;
  border-radius: 50%;
  border-style: solid;
  border-width: 0.1rem;
  border-color: gray;
}

Last step: make our radio change when checked

Again like the checkbox, using :checked we can style our radio button to make it appear different when selected.

label > input[type="radio"]:checked + *::before {
  background: radial-gradient(teal 0%, teal 40%, transparent 50%, transparent);
  border-color: teal;
}
label > input[type="radio"]:checked + * {
  color: teal;
}

The main difference between the radio button and the checkbox is the content and background. Here we have no difference in content and instead use a radial-gradient to fill only a portion of the circle in.

Going more in depth on the radial-gradient:

We start with teal 0%, since radial-gradient is defined from the center outward, we've basically said the center should start with teal.
Next teal 40% means "40% of the way to the edge should be teal". Since we started with teal, radial-gradient makes a gradient from teal to teal, also known as solid teal.
Then transparent 50% will cause a blending from teal to transparent between the 40 and 50 percent marks.
And the final transparent results in solid transparency until the edge.

Putting all that together, we've essentially made a solid teal dot occupying 40% of its element (with 5% of gentle fading away on each side).

Wrapping up and side notes

Thanks for reading! I like sharing and finding things like this where common web design patterns can be done without a massive JavaScript library or framework bogging the page down. Give me some suggestions below on patterns you'd like to see me break-down in CSS/HTML-only for this series.

Follow me for the next installment in this series: accordians!

While this example had "no unnecessary DOM"; it is also perfectly valid to include an additional span (or two) to hold svg/font-awesome icons for more precise/exotic radio button designs. Then :checked should be used to alternatively display: none and display: inline-block the icons and + will need to become ~ to select all the siblings.

Kallmanation