Traffic-lights with CSS

Just before Christmas I wanted to build a site that helped developers understand the impact that using Web Platform features would have on there potential reach. For example, if I used WebGL what is my target reach and what additional features can I use without impacting my potential audience figures.

I launched Sweet. Anyway, that is not the point of this post. One of the features of this site is.

One of the core features I wanted to implement was a traffic light system that graded the features like a traffic light. Green = Good, Red = Bad, Amber = Be Warned.

My original implementation was just using some simple CSS classes.

.good {
  color: green;

.ok {
  color: amber;

.bad {
  color: red;

I knew this was bad, but using RGB I couldn't work out sanely how to grade between 255,0,0, 255,126,0 and 0,255,0 without having logic in my code that looked like:

if (support < 25%) then .bad
if (support >25% and support < 75%) then .ok
if (support > 75%) then .good

added to that, I wanted the color to be more red than amber if it was at 35%.

Anyway, after a little chat with the awesome Paul Lewis, he mentioned that HSL (Hue, Saturation, Light) color scheme would be good for this problem because naturally the Hue value (0-359) rotates from Red, through Amber to Green (0 = Red, 45 = Amber, 90 = Green)

Red = color: hsla(0, 50%, 50%, 1) Amber = color: hsla(45, 50%, 50%, 1) Green = color: hsla(90, 50%, 50%, 1)

It is then simple given a value for percentage support to map that to 0-90 range and produce your traffic light colors.

element.styles.color = "hsla(" + ((percentage / 100) * 90) + ", 50%, 50%)";

About Me: Paul Kinlan

I lead the Chrome Developer Relations team at Google.

We want people to have the best experience possible on the web without having to install a native app or produce content in a walled garden.

Our team tries to make it easier for developers to build on the web by supporting every Chrome release, creating great content to support developers on, contributing to MDN, helping to improve browser compatibility, and some of the best developer tools like Lighthouse, Workbox, Squoosh to name just a few.