What are these CSS properties that start with a double-hyphen? [duplicate]

Question

I recently came across the website StuffAndNonsense.co.uk, which is a personal site for the digital designer Andy Clarke. I was astonished by it, using webpage technologies I didn't even know existed.

I looked into the styles and found this bit:

:root {
    --font-family: tondo_rg,sans-serif;
    --font-family-light: tondo_lt,sans-serif;
    --font-family-bold: tondo_bd,sans-serif;
    --font-weight: 400;
    --font-size-xxx-display: 8vmin;
    --font-size-xx-display: 4.11rem;
    --font-size-x-display: 3.653rem;
    --font-size-display: 3.247rem;
    --font-size-xxxx-large: 2.887rem;
    --font-size-xxx-large: 2.027rem;
    --font-size-xx-large: 1.802rem;
    --font-size-x-large: 1.602rem;
    --font-size-large: 1.424rem;
    --font-size-medium: 1.266rem;
    --font-size: 1.125rem;
    --font-size-small: 1rem;
    --font-size-x-small: .889rem;
    --font-size-xx-small: .79rem;
    --lineheight-text: 1.5;
    --lineheight-heading: 1.3;
    --color-background: #fff;
    --color-background-selection: #f0f2f3;
    --color-border: #cacfd5;
    --color-text-default: #0b1016;
    --color-text-alt: #95a0ac;
    --color-base: #f4f5f6;
    --color-accent: #ba0d37;
    --color-logo-enamel: #ba0d37;
    --color-logo-highlight: #fdfdfd;
    --color-logo-metal: #cacfd5;
    --color-logo-lettering: #fff;
    --color-logo-type: #0b1016;
    --color-text-link: #2c4159;
    --color-text-link-active: var(--color-text-link);
    --color-text-link-focus: var(--color-text-link);
    --color-text-link-hover: var(--color-accent);
    --color-text-link-visited: var(--color-text-link);
    --color-button-default: #2c4159;
    --color-button-alt: #243449;
    --color-button-border: #8586a4;
    --color-button-shadow: #ecedee;
    --border-radius-x-small: .25rem;
    --border-radius-small: .5rem;
    --border-radius-medium: .75rem;
    --border-radius-large: 1rem;
    --border-radius-circle: 50%;
    --border-width-hairline: .5px;
    --border-width-thin: 1px;
    --border-width-thick: 2px;
    --grid-gap: 4vw;
    --max-width: 92rem;
    --spacing-xx-small: .125rem;
    --spacing-x-small: .25rem;
    --spacing-small: .5rem;
    --spacing: .75rem;
    --spacing-medium: 1rem;
    --spacing-large: 1.5rem;
    --spacing-x-large: 2rem;
    --spacing-xx-large: 3rem;
    --duration-instantly: 0;
    --duration-immediately: .1s;
    --duration-quickly: .2s;
    --duration-promptly: .5s;
    --duration-slowly: 1s;
}

I was... confused to say the least. Not only had I never seen CSS properties prefixed by --, but I'd never seen stuff like font-size-xx-large: 1.802rem;. What is this doing? I tried Googling (and even Binging) it to no avail.

Answer 1

These properties are CSS Variables. You can see more on CSS variables here: https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables Here's an example of how to access CSS variables:

element {
background-color: var(--main-bg-color);
}

The reason they are in something called root: The :root selector allows you to target the highest-level "parent" element in the DOM, or document tree.

Answer 2

Variables in CSS starts with "--" which are case sensitive :root is used to define global scope which is similar to defining for body selector

:root {
  --font-size-xx-large: 16px;
  --font-size-xx-small: 12px;
}

For referencing that variable, we need to use var() function

    p {
      font-size: var(--font-size-xx-large);
    }

   div{
      font-size: var(--font-size-xx-small);
    }

Note: Currently it is not supported by all browsers

Check compatibility details here - https://caniuse.com/#feat=css-variables

code sample for reference - https://codepen.io/nagasai/pen/aYmPYv

:root {
  --font-size-xx-large: 16px;
  --font-size-xx-small: 12px;
}

p {
      font-size: var(--font-size-xx-large);
    }

div{
      font-size: var(--font-size-xx-small);
    }

<p>Paragraph font size large 16px</p>

<div>Div font size small 12px</div>