rgba

Switch font color for different backgrounds with CSS

Ever get one of those, "I can do that with CSS!" moments while watching someone flex their JavaScript muscles? That’s exactly the feeling I got while watching Dag-Inge Aas & Ida Aalen talk at CSSconf EU 2018.

They are based in Norway, where WCAG accessibility is not a just good practice, but actually required by law (go, Norway!). As they were developing a feature that allows user-selectable color theming for their main product, they faced a challenge: automatically adjusting the font color based on the selected background color of the container. If the background is dark, then it would be ideal to have a white text to keep it WCAG contrast compliant. But what happens if a light background color is selected instead? The text is both illegible and fails accessibility.

They used an elegant approach and solved the issue using the "color" npm package, adding conditional borders and automatic secondary color generation while they were at it.

But that’s a JavaScript solution. Here’s my pure CSS alternative.

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The Power of the rgba() Color Function in CSS

One of the things that I'm really interested in about CSS is the new color-mod function. It will give us the ability to do color manipulations right in the browser. For example, when hovering over a button, you can change the color by using something like color: color(black darkness(50%));, without the use of any CSS preprocessor like Sass.

But as the support of these CSS color functions is zero nowadays, we can temporarily use PostCSS and compile them as regular colors. Or we can experiment and discover the power of CSS rgba() color functions to change colors on the fly! Let's see how we can use it.

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RGBa Browser Support

This article was originally published on February 2, 2009 and is now being updated to clarify the concept and update the information for modern browsers.

RGBa is a way to declare a color in CSS that includes alpha transparency support. It looks like this:

div {
   background: rgba(200, 54, 54, 0.5); 
}

This allows us to fill areas with transparent color; the first thee numbers representing the color in RGB values and the fourth representing a transparency value between 0 and 1 (zero being fully transparent and one being fully opaque). We have long had the opacity property, which is similar, but opacity forces all decendant elements to also become transparent and there is no way to fight it (except weird positional hacks) Cross-browser opacity is also a bit sloppy.

With RGBa, we can make a box transparent and leave its descendants alone:

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